Rep. Daniel J. Flood (D.-Pa.) was unexpectedly called to the witness stand in U.S. District Court yesterday in a dramatic attempt by his lawyers to demonstrate that the ailing, 76-year-old congressman is mentally incapable of standing trial again on conspiracy, bribery and perjury charges.

The once flamboyant Flood, looking pale and expressionless, testified for more than 40 minutes, attempting to answer questions from defense lawyer Axel Kleiboemer about the issues in the government's case.

At one point, Flood weakly pounded his fist on the witness stand, after apparently struggling to recollect the penalties for conspiracy and bribery. "I know that. I know I knew that," Flood, a lawyer, told Kleiboemer.

Evidently surprised to find himself testifying, Flood at one point said to Kleiboemer, "I don't know why you called me. You didn't say you were going to call me." Later, during an interruption in his testimony, Flood asked, "What am I doing up here in the first place?"

Flood's slow, halting testimony came late yesterday during a hearing before Judge Oliver Gasch to determine if Flood is competent to be retried on the criminal charges. Flood is accused of taking more than $50,000 in bribes from several businessmen in exchange for his influence as chairman of a House Appropriations subcommittee controlling billions of dollars used for federal grants. His first trial ended in a hung jury last February.

Since then, Flood has been repeatedly hospitalized for a variety of illnesses, including gall bladder and cataract surgery. He was admitted to the Northern Virginia Doctors Hospital on Friday for treatment of ulcers and he returned there last night.

Flood, who was announced he will retire from Congress at the end of this month, is scheduled to appear for a retrial Feb. 25.

Yesterday, Flood was able generally to describe the meaning of an indictment and some of the allegations against him. Flood also summarized, again in general terms, the testimony of Stephen B. Elko, his former aide and the chief prosecution witness.

Earlier in the hearing, Dr. James L. Foy, a professor of psychiatry at Georgetown University Hospital, testified that Flood's memory and thought processes are impaired to the point where he would be unable to cooperate fully with his lawyers in preparing his defense.

Foy also recounted Flood's long history of drug dependency, which included use of sleeping pills, tranquilizers and sedatives. Although Flood was treated for drug withdrawal last month, Foy said he still remains agitated and preoccupied with his physical problems and continues to have memory problems.

When asked by Kleiboemer if Flood was pretending to have a poor memory, Foy said Flood's lapses are "so spotty" that it would be "extremely difficult" for someone to "fake it. . ."

Under cross-examination by government prosecutor Mark H. Tuohey III, Foy said that in his opinion Flood has "some mental capacity" to understand the proceedings against him but that his mental impairment would be a "serious handicap" in the preparation of a defense to criminal charges.

Kleiboemer is expected to resume questioning Flood today.