The Supreme Court ruled yesterday that the Federal Reserve System many continue, at least temporarily, to withhold from immediate public release its monthly decisions on monetary policy. The Fed claims an exemption under the Freedom of Information Act that no other federal agency has.

But the ruling left open the possibility of a later order to make the results of the Fed's Open Market Committee immediately open to the public. The usual practice has been to delay disclousure for about 30 days.

The government argued in the case that disruption of market conditions would be threatened by application of the Freedom of Information Act to the committee proceedings.

The Open Market Committee controls the supply of money, bank credit and bank reserves by buying and selling government and certain other securities in the domestic open market.

These purchases and sales are the most important policy instrument of the Federal Reserve. Increasing or decreasing reserves in the banks of the sellers or buyers affects the banks' ability to make loans and investments. This, in turn, makes a substantial impact on interest rates and investment activity in the economy as a whole.

After each monthly meeting, the FOMC - composed of seven members of the Fed's Board of Governors and five representatives of the Federal Reserve Banks - prepares a domestic policy directive.

The directive summarizes the committee's conclusions on the overall state of the economy and indicates its desire to follow an expansionary, deflationary or unchanged monetary policy in the period ahead. Since February 1977, the directive also has included specific tolerance ranges for growth in the money supply and the federal funds rate.

The directive and actions taken under it, involving in part transactions with about 25 securities dealers who trade primarily for their own account, are recorded in a document called a Record of Policy Actions.

The net result is that the directive makes its first public appearance when the record is published in the Federal Register. That's about a month after the directive was prepared. By that time, Justice Harry A. Blackmun wrote in the opinion for a 7-to-2 majority, "it has been supplanted by a new directive and is no longer the current and effective policy of the current and effective policy of the FOMC."

The month-long delays were challenged in 1975 when David R. Merrill, then a Georgetown University law student, filed a request under the Freedom of Information Act for very recent Records of Policy Actions. The Fed denied the requests, and Merrill sued. He won in a trial court and then in the U.S. Court of Appeals here.

The key issue was whether the documents were covered by the FOIA's Exemption 5, which covers "interagency or intra-agency memorandums or letters," and material that "would not be available by law to a party . . . in litigation with an agency."

The appellate court held that Exemption 5 didn't apply. Disagreeing, the Supreme Court held that it "incorporates a qualified privilege for commercial information" - including the directives.

They contain "sensitive information not otherwise available," Blackmun said. If immediate release would significantly harm the government's function or commerciaal interests, a "slight delay" is permitted, he said.

In the dissenting opinion, Justice John Paul Stevens termed this interpretation of Exemption 5 "incomprehensible."

Not until the case reached the Supreme Court did the Fed argue that immediate release of the directives would jeopardize the government's commercial interests by imposing additional borrowing costs on the government and that impacts of prompt disclosure are disputed by experts.

For that reason, Blackmun said, the appeals court decision is vacated and the case sent back for development of a "proper record." Stevens, whose dissent was signed by Justice Potter Stewart, said that the FOMC's operation "should be open to all - not just to a selected few."