The new law requiring federal agencies to hold their meetings in public goes into effect today, but some questions remain.

No one is quite sure, for example, what constitues a "meeting" under the law. Or what, precisely, is an "agency." Or how "public" the public sessions are required to be.

In a flurry of those wordy regulations that vex President Carter, about 48 executive agencies have offered a variety of answers to those questions as they gird themselves for a new dawn of public exposure under the "government-in-the sunshine" law.

Sen. Lawton Chiles (D-Fla.), who spent four years shepherding the "sunshine" bill through Congress, has been reading the agency regulations and is not entirely pleased with what he's found.

"They've put a lot of bramble bushes and pitfalls in the path," he said. "They may try to find or invent technicalities or nit-picking points of procedure . . . to close meetings."

The law, based on open-metings acts in several states, says that any federal office headed by a "collegial body= of two or more members appointed by the President must open its door whenever a majority of its members have a talk" that will "determine or result in" an agency decision.

Congressional debate on the bill made it clear that lawmakers thought they would be forcing numerous agencies - including such major ones as the Civil Aeronautics Board, the Federal Power Commission, the Nuclear Regulatory Commission, the Federal Communications Commissions and the Federal Trade Commission - to make their decisions in public.

But the agencies' regulations suggests that some of the sunshine may be eclipsed in pratice.

A few agencies seem more than willing to let the sun shine in.

The Foreign Claims Settlement Commission has offered to let regular observers reserve good seats at its meetings; those who drop in at the last minute also will be accommodated. The Commodity Futures Trading Commission has promised not to hold a meeting any place that discriminates on the basis of race, sex or ancestry.

But many agencies, including some of the biggest, seem to be adapting only grudgingly.

Agencies ranging from the Federal Deposit Insurance Corp. to the board of trustees of the Harry Truman Scholarship Foundation have decreed that people may attend meetings but may not take notes or pictures or record what transpires.

Others have suggested that "informal background sessions" or "gatherings of a social or ceremonial nature" will not be considered "meetings" even if conversations at such events night result in a decision.

The most ingenious dodge to date is reflected in the lengthy regulations issued by the Securities and Exchange Commission.

In those regulations, apparently for the first time, the SEC declares that any of its five commissiners can act as a "duty officer" in some cases and make a binding agency decision. Such decisions would be exempt from sunshine regulations, since the law applies only to "collegial" actions.

Some members of Congress, concerned that the new law does not touch the Cabinet agencies, tried to find a way to apply it to offices headed by a single decision-maker.

"But we just couldn't work out the logistics," said Eric L. Hirschhorn, a lawyer for the House government information subcommittee. "We would have had to put the Secretary's desk in an auditorium and made him think out loud."

Because the House subcommittee's chairman, Richardson Preyer (D-N.C.), has spent most of his time this year working on the House Assassinations Committee, Hirschhorn has emerged as the government's de-facto sunshine czar since the law was enacted six months ago.

Hirschhorn has taken a sten, pro-sunshine stance in dealings with affected agencies.

"We're telling these guys if the public isn't there they cann't talk about business," he says. "No more business lunch, no more staff briefings. They can talk about their golf games at lunch."

Hirschhorn's major battles have been with agencies that insist they are not covered by the new law.

Among them are the Legal Services Corp.; the Prevailing Rate Advisory Committee, which analyses blue-ctllar wage rates, and the Federal Reserve Board's Open Market Committee, which makes securities decisions that influence the money supply.

Many sunshine disagreements seem headed for the courts. "I hope we don't have a flood of suits," said Chiles. "But you're going to see a good deal of sunshine litigation."