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Northwestern Mutual Series Fund, Inc.

 Northwestern Mutual Series Fund, Inc.

720 East Wisconsin Avenue

Milwaukee, Wisconsin 53202-4797

 April 27, 2023

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 14, 2023, the Registrant filed with the Securities and Exchange Commission (“SEC”) Post-Effective
Amendment No. 86 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. 87 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 March 16, 2023 from Mr. Michael A. Rosenberg 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.

 Summary
Prospectus

1)
 Comment: With respect to the Growth Stock Portfolio, please explain what is meant by the term
“pecuniary” in the following sentence contained in the Principal Investment Strategies section of the Portfolio’s Risk/Return Summary: “The adviser integrates pecuniary environmental, social, and governance (ESG) factors into its
investment research process.”

 Response: Based on consideration of the Staff’s comment
and upon consultation with the sub-adviser to the Portfolio, Registrant will modify the disclosure language addressing the consideration of ESG factors set forth in the Principal Investment Strategies section
of the Portfolio Summary. The relevant disclosure language has been modified, in pertinent part, to read as follows (changes are redlined):

“The adviser integrates
pecuniary environmental, social, and governance (ESG) factors into its investment research process for certain investments. While ESG matters vary widely, the adviser generally considers ESG factors such as climate change,
resource depletion, labor standards, diversity, human rights issues, and governance structure and

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 Northwestern Mutual Series Fund, Inc.

720 East Wisconsin Avenue

Milwaukee, Wisconsin 53202-4797

practices. For certain types of investments, including, but not
limited to, cash, currency positions, and particular types of derivatives, an ESG analysis may not be relevant or possible due to lack of data. Where ESG considerations are integrated into the investment research process,
Tthe adviser focuses on the ESG factors that it considers most likely to have a material impact on the performance of the holdings in the Portfolio. The adviser may conclude that other attributes of an investment outweigh ESG considerations when making investment decisions
for the Portfolio. ESG factors that are considered include but are not limited to: exposure to energy transition, carbon footprint, climate action, product
sustainability, employee safety, income inequality and diversity.

Registrant advises that the sub-adviser to the Growth Stock Portfolio is T. Rowe
Price. T. Rowe Price also serves as sub-adviser to the Equity Income, Small Cap Value and Short-Term Bond Portfolios. Each of the Portfolios sub-advised by T. Rowe Price
relies upon a common ESG factor integration approach and utilizes disclosure language similar to that employed for the Growth Stock Portfolio. Registrant has made conforming changes related to the description of the consideration of ESG factors set
forth in the Principal Investment Strategies sections for each of the foregoing Portfolios.

2)
 Comment: With respect to each Portfolio which incorporates environmental, social and governance
(“ESG”) factors into its Principal Investment Strategies section, please include corresponding ESG risk disclosure as a separately identified Principal Risk. Alternatively, please explain why including Principal Risk disclosure related to
the consideration of ESG factors would not be appropriate. Additionally, for those Portfolios referencing the consideration of ESG factors as part of the Portfolio’s Principal Investment Strategies, affirmatively state if the sub-adviser to the individual Portfolio will be relying on data supplied by third-party service providers in connection with its ESG-related review process. If a sub-adviser relies upon data provided by third-party providers, please identify those third-party providers.

Response: The Registrant has added the following Principal Risk factor for each Portfolio which references the
consideration of ESG factors in its Principal Investment Strategies section:

 “ESG Risk – Incorporating
the consideration of ESG factors in the investment process may result in the exclusion of certain securities for non-investment reasons and therefore the Portfolio may forego some market opportunities
available to funds that do not consider ESG factors. The evaluation of ESG factors may affect the Portfolio’s exposure to certain issuers or industries and may not work as intended. The Portfolio may underperform other funds that do not
consider ESG factors or that use different ESG factors to evaluate a security. Information used to evaluate ESG factors may not be readily available, complete or accurate, and may vary across providers and issuers, as ESG factors are not uniformly
defined or assessed. There is no guarantee that evaluating ESG considerations for individual securities will positively contribute to the Portfolio’s performance results.”

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 Northwestern Mutual Series Fund, Inc.

720 East Wisconsin Avenue

Milwaukee, Wisconsin 53202-4797

 The Registrant notes that the adviser and certain sub-advisers may use and rely upon ESG-related information provided by third-party data providers. However, the Registrant respectfully declines to further revise or
enhance the disclosures related to ESG matters set forth in the Risk/Return Summaries for those Portfolios of the Fund that incorporate the consideration of ESG factors in order to disclose the use of such third-party providers. The Registrant
notes that the adviser and Portfolio sub-advisers may use and rely upon numerous third-party data and research providers as part of their security evaluation process. No disclosures are made with respect to
such other third-party data/research providers and Registrant believes no such disclosure is required or necessary as part of the Principal Investment Strategies disclosure. In addition, Registrant believes that including disclosure related to the
use of and reliance upon any third-party data/research providers related to ESG matters may place undue importance on the adviser’s or a sub-adviser’s consideration of ESG factors and/or the
extent to which the third-party data/research is relied upon and employed in connection with the security evaluation process. Further, Registrant notes that the ESG Risk disclosure set forth above describes potential risks related to information
utilized to evaluate ESG factors. Based upon the foregoing, Registrant believes the ESG-related disclosures, as modified as described herein, are appropriate and adequate.

3)
 Comment: The Staff notes that, for each Portfolio that includes a reference to the purchase of
American Depositary Receipts (“ADRs”) in the Principal Investment Strategies section, please indicate whether, in each case, the Portfolio may invest in unsponsored as well as sponsored ADRs, and whether the Portfolio’s transactions
in such ADR securities are conducted through exchanges or over-the-counter (“OTC”) markets. Additionally, in the corresponding Principal Risk factor disclosure
on ADR Risk, please address the risks of sponsored versus unsponsored ADRs, and transacting in such ADR securities through exchanges versus OTC markets.

Response: Registrant advises that references to the ability to invest in ADRs as a principal investment strategy appear
in fourteen Portfolios, as follows: (1) Growth Stock Portfolio; (2) Focused Appreciation Portfolio;
(3) Large Cap Core Stock Portfolio; (4) Large Cap Blend Portfolio; (5) Large Company Value Portfolio; (6) Equity Income Portfolio; (7) Mid Cap Growth Stock Portfolio; (8) Mid Cap Value Portfolio; (9) Small Cap
Growth Stock Portfolio; (10) Small Cap Value Portfolio; (11) International Growth Portfolio; (12) Research International Core Portfolio; (13) International Equity Portfolio; and (14) Emerging Markets Equity Portfolio.

 Registrant has modified the ADR Risk disclosure in the Principal Risks section of each Portfolio listed above to
address additional risks that may be posed by the purchase of unsponsored ADRs and the transacting market in which Portfolio ADRs may be traded, as applicable. The ADR Risk disclosure for the identified Portfolios has been modified to provide as
follows (changes are redlined):

 “ADR Risk – ADRs are receipts representing ownership of shares of a
foreign issuer held by a U.S. bank or similar financial institution that entitle the holder to dividends and capital gains on the underlying foreign shares. ADRs are alternatives to directly purchasing the

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 Northwestern Mutual Series Fund, Inc.

720 East Wisconsin Avenue

Milwaukee, Wisconsin 53202-4797

underlying foreign securities in their national markets and currencies. They are subject to many of the risks associated with direct investments in the foreign securities, such as currency
risk, political and economic risk and market risk, because their values depend on the performance of the non-dollar denominated underlying foreign securities. Investments in ADRs also subject a Portfolio
to fees and the credit risk of the financial institution holding the ADRs. ADRs may be “sponsored,” meaning
that they are implemented by a financial institution in collaboration with the issuing foreign company, or “unsponsored,” meaning that the financial institution created the instrument without the sponsorship or direct involvement of the
foreign company. Differing registration requirements apply to each type of ADR. ADRs may transact on exchanges or on over-the-counter markets (“OTC”) .
Conducting transactions in OTC markets may result in higher costs, a lack of pricing transparency and lower liquidity when compared with exchange-based transactions. Risks associated with different ADR types will vary, based upon differences in
registration, reporting and disclosure requirements that apply to such ADRs and the characteristics of the market in which transactions for the particular ADR are conducted.”

Registrant has additionally modified the disclosure in the “More About Principal Investment Strategies and Risks”
section of the Statutory Prospectus (changes are redlined):

 “ADR Risk. ADRs and similar depositary receipts
are subject to many of the same risks as direct investments in foreign securities, including the risk that material information about the issuer may not be disclosed in the United States and the risk that currency fluctuations
may adversely affect the value of the ADR. See “Risks of Foreign Investing” below. The financial institution may charge fees for forwarding dividends and interest and for other services. The Portfolios are also exposed
to the credit risk of the financial institution holding the ADRs. ADRs may be “sponsored,” meaning that
they are implemented by a financial institution in collaboration with the issuing foreign company, or “unsponsored,” meaning that the financial institution created the instrument without the sponsorship or direct involvement of the foreign
company. Differing registration requirements apply to each type of ADR. ADRs may transact on exchanges or on over-the-counter markets (“OTC”). Sponsored
ADRs may be established on three program levels, which differ with respect to market listing exposure and applicable reporting requirements. For Level 2 and Level 3 sponsored ADRs, the issuing financial institution and foreign company
jointly register the ADR with the Securities and Exchange Commission (“SEC”), after which the ADRs can be listed and traded on major U.S. stock exchanges. These ADR programs are subject to SEC registration requirements (including the
requirement to file an annual report and comply with U.S. accounting standards) and exchange listing requirements. Level 1 sponsored ADRs and unsponsored ADRs are exempt from full SEC registration and reporting requirements, and can only trade
on OTC markets. Conducting transactions in OTC markets may result in higher costs, a lack of pricing transparency and lower liquidity when compared with exchange-based transactions. Based on the foregoing, risks associated with different ADR types
will vary, based upon differences in registration, reporting, listing and disclosure requirements that apply to such ADRs and the characteristics of the market in which transactions for the particular ADR are conducted.”

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 Northwestern Mutual Series Fund, Inc.

720 East Wisconsin Avenue

Milwaukee, Wisconsin 53202-4797

4)
 Comment: The Staff notes that the Large Company Value Portfolio references the consideration of ESG
factors as part of the Portfolio’s Principal Investment Strategies section. The Staff indicates that the ESG-related disclosure for the Portfolio is vague and may be confusing to shareholders. Please
consider the ESG-related disclosure for this Portfolio and ensure that such disclosure explicitly describes the approach to the consideration of ESG factors as it relates to the Portfolio’s Principal
Investment Strategies and identifies ESG factors that are or may be considered.

 Response:
Respondent notes that the ESG-related disclosure language cited by Staff appears in the Principal Investment Strategies section for both the Large Company Value and Mid Cap Value Portfolios. (These Portfolios
are managed by Registrant’s sub-adviser, American Century Investment Management, Inc. (“ACI”).)

In response to the Staff’s Comment, and based upon consultation with ACI, Registrant has modified the current disclosure
set forth in the Principal Investment Strategies section of the Portfolio summaries. The disclosures have been modified to read as follows, in pertinent part (changes are redlined):

Large Company Value Portfolio

Mid Cap Value Portfolio

“In addition to fundamental financial metrics, the adviser may also consider environmental, social and/or governance
(ESG) data. However, the adviser may not consider ESG data with respect to every investment decision and, even when such data is considered, they may conclude that other attributes of an investment outweigh ESG considerations when making
decisions for the Portfolio. Examples of ESG data considered include, but are not limited to, carbon emissions, waste
and harmful substance management, product and employee safety, human capital management, and corporate governance structure and oversight.”

5)
 Comment: With respect to the Equity Income Portfolio and any other Portfolio that references the use
of REITs in its respective Principal Investment Strategies section, please include corresponding disclosure in the Principal Risks section of the Portfolio’s Risk/Return Summary addressing the risks posed by investing in REITs or otherwise
explain why such risk disclosure would not be appropriate.

 Response: Registrant advises that the
REITs Risk disclosure in the Principal Risks sections for the Equity Income, Small Cap Growth Stock and Mid Cap Growth Stock Portfolios had been deleted based upon Registrant’s review of each of those Portfolio’s holdings