Federal Reserve Chairman PAUL A. Volker yesterday proposed that money market mutual funds offering check-writing privileges be subject to reserve requirements similar to those of banks providing checking accounts.

Such a requirement could mean that investors in the money funds would be paid a lower interest rate than otherwise would be the case.

However, Treasury Secretary Donald T. Regan quickly announced his opposition to Volcker's proposal, saying such a move would do little to help the nation's hard-pressed thrift institutions. "I would rather see the thrifts and the commercial banks freed from some of the federal regulations that have hampered their competitiveness," Regan declared.

And officials of money market funds thought they already may have found a way to avoid the new restriction so that they could, in effect, continue to offer check-writing without forcing investors to sacrifice a portion of thier return.

Meanwhile a group of federal financial regulators, including Volcker and Regan, bucked the issue of aid to the troubled savings and loan industry back to Congress.

Action on three out of the four major issues on the Depository institutions Deregulation Committee agenda was postpones pending resolution on Capitol Hill several proposals that could help the thrifts, including creation of a tax-free savings certificate.

DIDC refused to consider for now restoration of the quarter-point interest rate advantage on money market certificates for thrifts, new short-term deposit instruments that would allow thrifts to compete more equally with money market mutual funds, and deregulation of Keogh and Individual Retirement Accounts.

However, the committee did put out for discussion a proposal to increase passbook account interest rates by 5 percentage points to 10 1/2 for thrifts and 10 1/4 percent for commercial banks. The regulator emphasized that they are neutral on the idea and are not necessarily advocating it.

The reserves which Volcker would have the money funds set aside must be placed in non-intrest-bearing accounts at the Federal Reserve. Since the money funds could not earn a return on that portion of their assets, investors in the money funds would have to get a lower return, too.

Volcker, testifying before the a House Banking subcommittee, said his request, which would require legislation, probably would mean that the money market funds would set up two types of accounts: one with check-writing privileges, required reserves and thus a lower return; and the other without such privileges and therefore no implicit interest penalty.

Thrift institutions have complained bitterly that the higher returns offered by the swiftly growing money funds have sucked cash out of the thrifts, adding greatly to their current woes.

"Money market mutual funds have an artificial and continuing competitive advantage, so long as interest is not paid on reserve balances," Volcker told the banking subcommittee.

Assets of the money funds jumped another $2.67 billion in the week ended June 24 to a total of $124.75 billion as investors responded to 17 percent-plus yields, according to the Investment Company Institute.

Meanwhile, the nation's savings and loan associations, batered by record withdrawals in March and April, suffered an unprecedented third straight savings loss in May, federal regulators said yesterday.

U.S. savers withdrew $161 million more than they deposited in May, also marking the first time the federally insured S&Ls had ever suffered a net savings outflow in that month. However, May's decline was far short of the losses of $2.1 billion in March and $4.6 billion in April, the Federal Home Loan Bank Board said.

At present, investors most of the money funds can write checks against their investment just like against a checking account. The checks usually must be for some minimum amount -- say, $500 -- and they can be made payable to third parties. Most, however, are made payable to the investor himself and deposited in regular bank accounts, studies have shown.

While the checks nominally are written against a money fund, they actually are cleared by the commercial bank handling the fund's business. Some fund officials believe they could circumvent Volcker's reserve requirement simply by having the checks written against the bank with funds provided through a so-called zero-balance account. Merrill Lynch clears checks drawn on its money management accounts in this fashion through a bank in Columbus, Ohio. Since the account always has a zero balance at the end of the day, technically no funds are on deposit and therefore the reserves required are also zero.

The DIDC also sped up the pace of banking deregulation. Instead of the original six-year phase-out, the committee agreed to a Regan suggestion for a four-year deregulation plan with the proviso that it could be hastened or retarded as economic conditions warrant.

Rate ceilings on all deposists of four years or more would end on Aug. 1. The rate ceilings for deposits of 2 1/2 to 4 years would be pegged to the 2 1/2-year Treasury security, currently 14.45 percent. Thrifts could pay the Treasury rate, banks would pay a quarter of percentage point less, and both would be allowed to compound interest.

During the second year, the ceilings on 3- to 4-year deposits would be eliminated, and this would continue annually until all ceilings and all pegs to Treasury securities and the differential for all certificates have been removed.