Dr. Murdock Head, the dapper, erudite director of the Airlie Foundation in Warrenton, Va., allegedly picked up an envelope containing $5,000 in $100 bills from his deak, using facial tissue to avoid leaving fingerprints.

Then, according to his accusers, he handed the unsealed and unmarked envelope to a congressional aide, saying "Give this to Congressman Flood."

This incident in April 1971, allegedly was among the first of a succession of payoffs for which Head, 55, is scheduled to go on trial today.

He has been charged with conspiring to arrange $49,000 in bribes to Rep. Daniel J. Flood (D-Pa.), former Flood aide Stephen B. Elko, and former Rep. Otto E. Passman (D-La.) in exchange for their help in obtaining lucrative federal contracts and grants for his foundation. He is also accused of secretly arranging an illegal $11,000 loan to a now-retired Internal Revenue Service agent in return for favorable tax treatment.

The government's efforts to prosecute Head were unexpectedly thrown into doubt Friday afternoon by a judge's order limiting key evidence that could be presented at Head's trial. Prosecutors later said that the ruling "would have the effect of destroying the government's case" against Head, unless the order were modified. But yesterday, Judge Albert V. Bryan Jr. agreed to revise his decision, and prosecution evidence will not be so severely restricted.

The trial in a federal court in Alexandria is expected to offer a glimpse into what prosecutors portray as blatant -- though frequently highly sophisticated -- corruption among the Washington area's powerful elite. It is likely to contain echoes of past courtoom dramas, including the first bribery trial of Rep. Flood, which ended in a mistrial last February. Flood's retrial is currently set for November.

Head's trial has already stirred considerable legal controversy. Federal prosecutors, in a highly unusual move, urged the U.S. Court of Appeals in Richmond to remove Judge Oren R. Lewis from presiding at the trial. They complained that Lewis had displayed an "angry attitude" and "an appearance of bias" against the prosecution. The appeals court rebuffed their move.

Lewis now appears likely to preside at Head's trial. This was disclosed by Bryan at the end of yesterday's hearing although lawyers and court officials cautioned that last-minute changes in plan sometimes occur.

The Texas-born Head, an administrator and scholar who holds graduate degrees in medicine, dentistry and law, runs a tax-exempt foundation that sprawls over more than 1,700 acres of gently sloping farmland in the foothills of the Blue Ridge Mountains about 50 miles west of Washington. He is also a George Washington University professor and chairman of the university's department of medical and public affairs.

Head has pleaded innocent to the conspiracy, bribery, tax-evasion and other charges, which were returned after a protracted investigation by a grand jury that obtained testimony from some prominent Washington figures, including former Health, Education and Welfare Secretary Elliot L. Richardson and former Defense Secretary Melvin R. Laird. There has been no indication, however, that Laird, a close friend of Head, or Richardson will testify at Head's trial.

Head, slender, atheltic and reclusive, has viewed the controversy surrounding his 19-year-old Warrenton conference center and retreat with a mixture of characteristic optimism and some less characteristic fatalism.

"Although we will weather this storm, there is little we can do but watch it run its course," he said, not long after the allegations first surfaced.

Head's lawyers, Brian P. Gettings and Frank W. Dunham Jr., have indicated that part of the defense strategy will likely be to call Head to the witness stand in an attempt to attack the credibility of a key prosecution witness -- Elko, Flood's former aide.

Elko, who was also a key prosecution witness in Flood's first trial, has been convicted of perjury and other charges in a bribery scheme involving a California vocational school and is currently on parole from a two-year prison sentence. He has been granted immunity in exchange for his testimony.

It is Elko who described the April 1971 meeting at which Head allegedly used facial tissue to avoid getting fingerprints on the envelope containing the $5,000 bribe for Flood. Elko testified about this and other incidents during the Flood trial.

At a later meeting with Head at which another $5,000 bribe allegedly changed hands, Elko said: "I kidded him about this facial tissue business, and he said he did not want any fingerprints on the envelope."

Elko depicted a subsequent payoff session at Airlie Foundation headquarters in more sinister terms. December 1973, he said, he met with Head in the foundation's film screening room, where Head allegedly spoke in a whisper and wrote messages on an easel. "We have to be careful. Long knives are out," Elko said Head wrote on the easel.

Then, Elko testified, they bargained over the amount of the next bribe. Elko said he wrote $15,000 on the easel. Then, he testified, Head wrote "too much" and suggested $5,000. Finally, Elko said, he proposed $8,000 and Head wrote "agreed." Afterwards, Elko testified, Head burned the three sheets of paper on which they had written these messages in a fireplace.

Similar glimpses of Head's alleged financial dealings were also provided during the Flood trial by other witnesses who have been subpoenaed to testify against Head.

Charlotte Fowler, Head's former executive assistant, recalled keeping thousands of dollars in a drawer and periodically cleaning it off before turning it over to Head. "I had a pair of white gloves in my desk," she testified. "I would put on the gloves and I would wipe the money on both sides. I would put it in a plain white envelope."