When the Office of Government Ethics reminded executive branch employes in October that it was a violation of regulations to allow lobbyists, businessmen or reporters to pick up the tab for one-on-one meals, some predicted the end of expense-account civilization as many inside the Beltway have known it.

The clampdown, which does not apply to the legislative and judicial branches, has been mixed in the executive departments, however.

The big casualties appear to be the White House staff and the higher-priced restaurants where journalists and lobbyists took them to lunch.

Tony Greco, owner of the Maison Blanche, which is just a block from the White House and the Old Executive Office Building, estimates his lunch trade is off about 10 percent in the last two months.

"My fellow restaurateurs agree, it started gradually but now it's serious," he said. "Media people are probably 40 percent of my lunch business. Hey, this is restricting freedom of the press. How do you get government whistle-blowers to blow the whistle if you don't buy them lunch or dinner? They won't do it in their offices."

In some agencies, however, officials seem to be ignoring the rule and dining as usual. These officials seem to be grouped in the diplomatic and national security fields, for the most part.

Other agencies, particularly the regulatory ones, report no change because they've long had stringent regulations prohibiting acceptance of such meals. And many bureaucrats, in the words of one reporter who covers the Department of Education, "never get asked to lunch."

The meal prohibition is based on a 1965 executive order, which was aimed at preventing government employes from benefiting from their positions and avoiding the appearance of wrongdoing.

The order forbids most government officials, from the president to typists and custodians, from accepting anything of value, including expense account meals, from "prohibited sources." This includes journalists, lobbyists, corporate representatives and firms that do business with or are regulated by the government.

Some government officials contend that the real losers are the reporters, businessmen and lobbyists whose lunches were free when they dined with government officials because they could put them on the expense account as business activities.

There briefly appeared to be the making of a crisis when it was feared that such traditions as the annual Gridiron Club and White House Correspondents' Association (WHCA) dinners might be imperiled. The president, top government officials and legislative and political leaders are traditionally invited by members of the two groups.

These dinners, however, have been deemed to be within the regulation's bounds, although the Gridiron Club changed its method of inviting executive branch guests: they are now guests of the club, not of individual members, as before.

"It was the overwhelming judgment of both the outgoing and incoming boards of directors that we would not have been in violation because it's the institution that puts the dinner on," said James H. McCartney of Knight-Ridder, the Gridiron president. "We've been having the dinner for 102 years and we're not aware that anyone has been corrupted by it but we wanted to act within the spirit."

Norman Sandler of United Press International, president of the WHCA, agreed. "We shouldn't be affected in the least because there is an exception for 'widely attended gatherings,' " he said.

But one-on-one meals with anyone who is not a personal friend -- and therefore can't put them on an expense account or take a tax deduction for them -- is taboo for White House personnel.

"We decided right after the advisory came out in October that if a lunch is designed to get information from us, it's prohibited," White House spokesman Marlin Fitzwater said.

The effect was dramatically noted when President Reagan spent the Thanksgiving holiday on his ranch near Santa Barbara, Calif. Instead of taking administration officials to lunch at places like the Biltmore Hotel or El Encanto restaurant, White House reporters and their sources wound up dining Dutch treat at an unprepossessing Mexican restaurant where, as one correspondent put it, "it's impossible for one person to eat $5 worth of food."

In some cases, an official gets around the ruling by letting the journalist or lobbyist pick up the tab with the agreement that he'll get it the next time -- which never happens.

However, reporters said that officials of some agencies, such as the National Transportation Safety Board, are scrupulous about splitting the check or taking turns picking it up.

"We've always had rules against one-on-one lunches and understood that the way the executive order read meant that we couldn't accept anything from anyone regulated by our agency," said Myrna G. Siegel, who is with the solicitor's office in the Securities and Exchange Commission.

Some departments, such as State and Treasury, have special problems.

"We have always basically followed the rules but Treasury, like State, has to consider things like embassy invitations and gifts of nominal value, which are all right to accept under the Gifts and Decorations Act of 1978," said Miklos Lonkay of the general counsel's office. "We're a policy-making department and have stringent regulations for a vast portion of it. Internal Revenue Service, for instance, is extremely careful about appearances."

"Our policy all along has been not to let outsiders pick up the check but many of our people didn't realize it," said Alan J. Whitney, a lawyer and ethics adviser with the Federal Deposit Insurance Corp. "Probably only a dozen or so people are affected by this reminder. In the past, I took turns and wouldn't allow a reporter or someone else to pick up the check unless I knew there would be the opportunity to reciprocate. I still have lunch with the same people."