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Northwestern Mutual Series Fund, Inc.

 Northwestern Mutual Series Fund, Inc.

720 East Wisconsin Avenue

Milwaukee, Wisconsin 53202-4797

April 16, 2025

 VIA EDGAR

U.S. Securities and Exchange Commission

100 F Street, N.E.

 Washington,
D.C. 20549

 RE:  Northwestern Mutual Series Fund, Inc. (“Registrant”)

   Registration Nos: 2-89971; 811-3990

   EDGAR CIK No. 0000742212

Ladies and Gentlemen:

 On
February 21, 2025, the Registrant filed with the Securities and Exchange Commission (“SEC”) Post- Effective Amendment No. 91 to the Registrant’s registration statement on Form N-1A
(the “Registration Statement”) under Rule 485(a)(1) of the Securities Act of 1933, as amended (the “Securities Act”), which filing was also submitted as Amendment No. 92 to the Registrant’s Registration Statement under
the Investment Company Act of 1940, as amended (the “1940 Act”). The Registration Statement includes the offering documents for the Registrant’s currently effective 27 series (each, a “Portfolio” and together, the
“Portfolios”).

 The primary purpose of this filing is to respond to oral comments received on April 7, 2025 from Mr. Aaron
Brodsky of the SEC staff (the “Staff”) on the Registration Statement.

 The following paragraphs provide the Registrant’s
response to comments received. Each comment has been repeated below in the order received, and the Registrant’s response follows. We hereby confirm that all changes discussed in this letter will be incorporated in the Registrant’s
subsequent post-effective amendment filing under Rule 485(b) (the “485(b) filing”). We also confirm that if disclosure is revised in the 485(b) filing in response to a comment from the Staff, we will make corresponding changes to the
disclosure in other applicable locations within the 485(b) filing.

1)
 Comment: For each Portfolio, please provide a completed fee table and expense example for each
Portfolio.

 Response: The Registrant confirms that the completed fee table and expense example
for each Portfolio will be delivered to the Staff via a separate communication.

2)
 Comment: With respect to the Growth Stock Portfolio, and elsewhere as applicable, please add
disclosure to describe how a third-party growth index is defined.

 Response: The Registrant will
revise the disclosure for the Growth Stock Portfolio as follows:

 “Growth companies, as defined by the adviser, are
those included in a third-party growth index. For this purpose, a third-party growth index is an index developed, calculated, and
maintained by a third-party that measures the performance of growth stocks.”

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 The Registrant advises that the use of the term “third-party growth
index” is also employed by the Mid Cap Growth Stock Portfolio, the Small Cap Growth Stock Portfolio, and the International Growth Portfolio. The Registrant will also revise the disclosure for each listed Portfolio to add the sentence indicated
to describe how a “third-party growth index” is defined.

3)
 Comment: With respect to the Growth Stock Portfolio, if the Portfolio is intending to track an index,
please add disclosure indicating this and disclose if the Portfolio will attempt to replicate the index or use representative sampling.

Response: The Registrant confirms supplementally that the Portfolio does not intend to track an index.

4)
 Comment: With respect to the Growth Stock Portfolio, the Staff notes that the disclosure in
“Large Position Risk” states that the Portfolio “may hold and maintain large positions in a single or small number of securities.” Please explain supplementally (i) whether any such securities are foreign, and if so what
information is publicly available and (ii) when the Portfolio holds such large positions, how the Portfolio expects its assets to be allocated.

Response: The Registrant notes that (i) the Portfolio currently does not hold any large positions in foreign
securities, and (ii) the Portfolio’s assets are allocated in a manner that complies with its principal investment strategies as described in the Prospectus, its investment restrictions as disclosed in the Statement of Additional
Information, and the requirements of the 1940 Act and rules thereunder.

5)
 Comment: With respect to the Growth Stock Portfolio, and elsewhere as applicable, please add
disclosure to the “Principal Investment Strategy” section to indicate the potential focus on a small number of securities.

Response: The Registrant will revise the disclosure as indicated below in the response to Comment #6. In addition, the
Registrant confirms supplementally that the indicated disclosure will be added to the “Principal Investment Strategy” of each of the Large Cap Core Stock Portfolio and the Large Cap Blend Portfolio.

6)
 Comment: With respect to the Growth Stock Portfolio, and elsewhere as applicable, the Staff notes
that “Mid Cap Company Risk” is included as a Principal Risk of the Portfolio. Please add corresponding disclosure to the Principal Investment Strategy.

Response: The Registrant will revise the disclosure for the Growth Stock Portfolio as follows:

“The adviser generally selects “blue chip” growth companies, which are, in the adviser’s view, well
established in their industries and have the potential for above-average earnings growth, for inclusion in the Portfolio. The investment approach reflects the adviser’s belief that solid company fundamentals (with an emphasis on the potential
for strong growth in earnings per share or operating cash flow) combined with a positive outlook for a company’s industry will ultimately reward investors with strong investment performance. The Portfolio normally invests in stocks of large and mid capitalization companies. While the Portfolio does not seek to
focus its investments in any particular sector, the Portfolio may at times have a relatively high percentage of its assets invested in a particular sector as a result of the adviser’s stock selection process. Further, the Portfolio may hold and maintain large positions in a single or small number of securities as a result of the adviser’s investment strategy.

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 The Registrant also confirms supplementally that it will add similar
corresponding disclosure regarding mid-capitalization companies to the Principal Investment Strategy of Large Cap Blend Portfolio.

7)
 Comment: With respect to the Focused Appreciation Portfolio, please disclose what the term
“Focused” means in the context of the Principal Investment Strategy.

 Response: The
Registrant will revise the disclosure as follows:

 “The adviser utilizes a “focused” investment strategy resulting in the Portfolio
investings
primarily in a core group of 30-40 securities., butThe Portfolio may at times exceed this range.

8)
 Comment: With respect to the Large Cap Blend Portfolio, please disclose what the term
“Blend” means in the context of the Principal Investment Strategy.

 Response: The
Registrant will revise the disclosure as follows:

 “The Portfolio focuses invests in a “blend” ofon those equity securities of
companies with growth and value characteristics that it considers attractively valued and seeks to
outperform produce returns that exceed those of the benchmark through superior stock selection. By emphasizing attractively valued equity securities, the Portfolio seeks to
produce returns that exceed those of the benchmark.

9)
 Comment: With respect to the Large Company Value Portfolio, and elsewhere as applicable, please
disclose what “similar to” means with respect to the capitalization range identified.

Response: The Registrant will revise the disclosure for the Large Company Value Portfolio as follows:

“For purposes of the Portfolio, large-capitalization companies are those companies with market capitalizations within the capitalization range of
similar to companies in the Russell 1000®
Index.

 The Registrant advises that the use of the phrase “similar to” to describe a capitalization range
is also employed by the Mid Cap Growth Stock Portfolio, the Mid Cap Value Portfolio, and the Small Cap Growth Stock Portfolio. The Registrant will revise the disclosure of each such Portfolio as indicated above.

10)
 Comment: With respect to the Large Company Value Portfolio, and elsewhere as applicable, please add
disclosure to describe how a third-party value index is defined.

 Response: The Registrant will
revise the disclosure for the Large Company Value Portfolio as follows:

 “Value companies, as defined by the
adviser, are those included in a third-party value index or those that exhibit certain financial characteristics (e.g., price-to-earnings ratio, price-to-book ratio, enterprise value-to-EBITDA ratio, cash flow trends or dividend payment
history and projections) determined by the adviser to indicate the company is undervalued by the market). For this purpose, a third-party value index is an index developed,
calculated, and maintained by a third-party that measures the performance of value stocks.

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 The Registrant advises that the use of the term “third-party value
index” is also employed by the Mid Cap Value Portfolio and the Small Cap Value Portfolio. The Registrant will also revise the disclosure for each listed Portfolio to add the sentence indicated to describe how a “third-party value
index” is defined.

11)
 Comment: With respect to the Mid Cap Growth Stock Portfolio, please revise the disclosure to explain
the lower boundary of the capitalization range for purposes of the Portfolio’s 80% investment policy.

Response: The Registrant will revise the disclosure as follows:

“For purposes of the Portfolio, mid-capitalization companies are those companies
with market capitalizations similar to companies inwithin the
capitalization range of the Russell MidCap® Growth Index. As of March 31, 2025, this range was approximately [$662272.604 million to
$19756.6695 billion].

12)
 Comment: With respect to the Mid Cap Growth Stock Portfolio, the Staff notes the disclosure
describing growth companies including that “the company has above-average growth potential.” Please add disclosure to define “above-average growth potential.”

Response: The Registrant will revise the disclosure as follows:

“Growth companies, as defined by the adviser, are those included in a third-party growth index or those that exhibit
certain financial characteristics (e.g., a company’s forward and historical earnings growth, or its forward and historical revenue growth) determined by the adviser to indicate the company has
above-average growth potential (e.g., sales, revenue, or earnings growth that may outpace peer averages or the overall
market).”

 The Registrant advises that the use of the
term “above-average growth potential” is also employed by the Small Cap Growth Stock Portfolio and the International Growth Portfolio. The Registrant will also revise the disclosure for each listed Portfolio to add the disclosure indicated
in the appropriate location.

13)
 Comment: With respect to the Index 400 Stock Portfolio, “Exchange-Traded Fund Risk” is
included as a Principal Risk of the Portfolio. Please either add corresponding disclosure to the “Principal Investment Strategies” section or remove the identified risk from the “Principal Risks” section.

Response: The Registrant notes that the existing “Principal Investment Strategies” section contains
corresponding disclosure describing the Portfolio’s ability to invest in exchange-traded funds, as reproduced below, and accordingly respectfully declines to revise the disclosure in response to this comment.

“The Portfolio may also invest in equity index futures and exchange-traded funds for cash management and liquidity
purposes and to help achieve full replication.”

14)
 Comment: With respect to the Small Cap Growth Stock Portfolio, please revise the disclosure in the
“Principal Investment Strategies” section relating to the Russell 2000® Growth Index as that index is not considered a broad-based index securities market index.

 Response: The Registrant will revise the disclosure to indicate that the identified index is not
the Portfolio’s broad-based securities market index as follows:

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 “The Portfolio is structured so that its sector weights are generally
similar to those of the Russell 2000® Growth Index, the Portfolio’s benchmarkan index that aligns with the Portfolio’s strategy.”

15)
 Comment: With respect to the International Growth Portfolio, please delete the references to American
Depositary Receipts and American Depositary Shares from the description of countries outside the United States.

Response: The Registrant will make the requested change.

16)
 Comment: With respect to the Long-Term U.S. Government Bond Portfolio, please supplementally explain
the inclusion of the line item in the fee table for “Interest Expense.” Please also confirm that the Portfolio’s interest expense is not counted twice in the calculation of Total Annual Portfolio Operating Expenses.

 Response: The Registrant notes supplementally that the “Interest Expense” line item
relates to the Portfolio’s financing transactions utilizing various U.S. Treasury bonds (i.e., Treasury rolls). Further, the Registrant confirms that the Portfolio’s Total Annual Portfolio Operating Expenses properly accounts for the
interest expense, and that the fee table to be submitted in response to Comment #1 will reflect such expense.

17)
 Comment: With respect to the Long-Term U.S. Government Bond Portfolio, please add disclosure to
describe how “government sponsored enterprises” are defined.

 Response: The Registrant
will revise the disclosure as follows:

 “Normally, the Portfolio invests at least 80% of net assets (plus any
borrowings for investment purposes) in debt securities that are issued or guaranteed by the U.S. Government, its agencies or government sponsored enterprises (“U.S. Government Securities”). Government sponsored enterprises include, for example, the Federal National Mortgage Association (Fannie Mae), the Federal Home Loan Mortgage Corp. (Freddie Mac), the Federal
Home Loan Bank (FHLB), the Farm Credit System (FCS) and the Federal Agricultural Mortgage Corp. (Farmer Mac).

18)
 Comment: With respect to the Long-Term U.S. Government Bond Portfolio, please revise the disclosure
relating to the 80% investment policy to indicate that derivatives will be used to provide exposure to U.S. Government debt securities.

Response: The Registrant will revise the disclosure as follows:

“U.S. Government Securities may be represented by forwards or derivatives such as options, futures contracts or interest
rate swap agreements (to take a position on interest rates moving either up or down) that provide exposure to U.S. Government Securities.”

19)
 Comment: With respect to the Long-Term U.S. Government Bond Portfolio, please revise the disclosure
regarding investing all of its assets in derivatives instruments to clarify that such investments are subject to the Portfolio’s 80% investment policy.

Response: The Registrant will revise the disclosure as follows:

“The Portfolio may invest all of its assets in derivative instruments, such as options, futures contracts or interest
rate swap agreements (to take a position on interest rates moving either up or down), or in mortgage- or asset-backed securities, subject to the Portfolio’s objective and the Fund’s policies including the 80% investment policy.

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20)
 Comment: With respect to the Inflation Managed Portfolio, please revise the footnote to the fee table
(i) to indicate that the termination date of the fee waiver agreement is at least one year from the effective date of the Registration Statement and (ii) disclose who can terminate the fee waiver agreement.

Response: The Registrant confirms that (i) the date disclosed will be at least one year from the effective date of
the Registration Statement and (ii) disclosure will be added (and included in the fee table delivered in response to Comment #1) to note that the fee