When the guilty verdicts were returned against former White House deputy chief of staff Michael K. Deaver at the U.S. Courthouse here yesterday afternoon, another old friend of President Reagan's, Lyn Nofziger, was sitting at a defense table in another courtroom just down the hall for a hearing on the influence-peddling charges confronting him.

One flight up, on the grand jury floor, Attorney General Edwin Meese III was winding up his fifth interrogation before the grand jury investigating the Iran-contra affair. Earlier in the day, Assistant Secretary of State Elliott Abrams testified under oath before the same panel.

The ethics of the Reagan administration are under siege. More than 110 senior officials have been accused of unethical or illegal conduct since Reagan took office in January 1981, a number that does not include those involved in the Iran-contra affair or the Wedtech scandal.

The allegations ranged from lying to Congress to questionable stock deals, from favors for relatives and friends to defrauding the government, from illegal lobbying to inadequate financial disclosures.

Before passage of the 1978 Ethics in Government Act, many of the charges might not have been prosecuted. But for high-ranking officials at least, the independent counsel provisions of the law now require the assignment of court-appointed prosecutors to guard against such deference to power.

The law, as congressional committees that revised it this year pointed out, has not always worked as it was designed. There were only two convictions under it, both the result of prearranged guilty pleas, before Deaver was prosecuted. The verdicts against him yesterday afternoon are the first jury trial to be conducted under the independent counsel law.

There is more to come. Nofziger, the former White House political adviser, is scheduled to be tried here next month with his lobbyist-partner Mark Bragg on charges of violating federal ethics laws in lobbying the administration on behalf of the Wedtech Corp. and other clients soon after Nofziger left the White House in 1982.

The criminal investigation of the Iran-contra affair, being conducted by independent counsel Lawrence E. Walsh, also is expected to result in wide-ranging indictments in late January or perhaps February. The targets include a former National Security Council aide, Lt. Col. Oliver L. North; a former White House national security adviser, retired rear admiral John M. Poindexter; retired Air Force major general Richard V. Secord and his business partner, arms dealer Albert Hakim.

Critics have charged that Reagan has encouraged an "anything goes" atmosphere by failing to condemn influence-peddling and other misdeeds. Instead, he has usually taken the position that no wrongdoing took place.

At the same time, Reagan and the Justice Department have been outspoken in their denunciations of the independent counsel law, assailing it as unconstitutional and unnecessarily costly. Reagan vigorously voiced his complaints Tuesday when he reluctantly signed the reauthorization act extending the system for five more years.

Congressional backers of the law, who had been expecting a veto, quickly concluded that Reagan signed it because he knew a veto would have been overridden. Old friends and advisers such as Nofziger were disappointed.

"If it's a lousy law, then don't sign it," Nofziger said as he walked into the courtroom for yesterday's pretrial skirmish with independent counsel James C. McKay. "This idea of saying it's a lousy law, but I'm going to sign it anyway never made sense to me."

Besides Deaver, other Reagan administration officials who have been convicted include:Rita M. Lavelle, former head of the toxic waste cleanup program at the Environmental Protection Agency, who was found guilty in 1983 of lying to Congress and obstructing a congessional inquiry. C. McClain Haddow, chief of staff to former health and human services secretary Margaret M. Heckler, who pleaded guilty this fall to taking $55,330 from a nonprofit foundation he started while working for the government. Paul Thayer, former deputy secretary of defense, who pleaded guilty in 1985 to obstructing justice and giving false testimony in an insider trading scheme.

Others were investigated without any charges being brought. Some were tried and acquitted. Former secretary of labor Raymond J. Donovan was investigated three times by a special prosecutor who found insufficient evidence to charge him with any federal crime, then was tried on fraud and grand larceny charges in a New York state court and acquitted last May.

According to statistics compiled by a Senate Governmental Affairs subcommittee headed by Carl M. Levin (D-Mich.), there are also far more cases processed at the Justice Department under the independent counsel law than have come to light in any detail: a total of 36 since 1982 when the statute was last revised to give the attorney general more leeway in sifting allegations.

Of the 36 cases, the Justice Department closed 25 without ordering a preliminary Federal Bureau of Investigation probe, as the law specifies.

It reported closing five of these cases because the allegations did not involve a high-ranking "covered official," but 20 others because a "threshhold inquiry" in the Justice Department had determined that the information was insufficient to warrant calling in the FBI.

Congress took steps to close such loopholes in the reauthorization measure that became law this week. Reagan's comment: "H.R. 2929 {the reauthorization bill} contains a number of new provisions that aggravate the infirmities in existing law."