Members of the congressional Iran-contra committees, wrapping up more than 11 weeks of public hearings, said yesterday they are still not sure which witnesses to believe, but asserted that the essential lessons of the episode are clear.

The committee members expressed particular puzzlement over whether the late CIA Director William J. Casey, whose role in the affair was the subject of sharply conflicting testimony, knew of the diversion of arms sale profits to the Nicaraguan rebels.

"I'm in a dilemma like most Americans," Senate Chairman Daniel K. Inouye (D-Hawaii) told reporters after gaveling the hearings closed. "Do I believe the admiral {former national security adviser John M. Poindexter} or the colonel {fired White House aide Oliver L. North}, or do I believe statements attributed to Mr. Casey? Do I believe the secretary of state or do I believe the secretary of defense?"

But the committee members said that contradictions in the testimony do not stand in the way of understanding the essential facts of the affair and should not prevent the country from drawing valuable lessons.

"{It's true} when you have thousands of pages of testimony that people will have different views of the truth and recollection," said Senate Vice Chairman Warren B. Rudman (R-N.H.). "But I don't think they affect the story."

Committee members interviewed after the hearings ended yesterday proposed changes ranging from screening top government officials more carefully to fundamentally reexamining the intent of the Constitution.

"Every major breakdown in this country's history, whether it be military or political, can usually be traced to the erroneous judgments of people -- not process, not of law," said Rudman, calling for more stringent evaluation of officials for "judgment, experience and balance."

Inouye suggested that all Americans reflect on the period of "stress and strain and turmoil" when the Constitution was drafted 200 years ago. "That period was much more dangerous than the one we have today," Inouye said, "and yet at that time they decided that they did not want an autocratic, secret government."

Most of the lawmakers agreed that the hearings uncovered no direct evidence that President Reagan knew of the diversion of Iranian arms sales profits to aid the Nicaraguan contras.

But Sen. Paul S. Sarbanes (D-Md.) told WJLA-TV that testimony by Poindexter suggests that a cover-up was plausible. "I think you have to see his testimony through the prism of 'deniability,' which he himself said was the overriding consideration to him," Sarbanes said.

Poindexter, in his testimony two weeks ago, told the committees that he kept the existence of the affair from the president so that Reagan could deny involvement and escape political damage from the controversial program.

Sen. George J. Mitchell (D-Maine), in an interview with CBS News, said he believed that Reagan did not know of the affair but charged that committee members regard Poindexter as the least credible witness of any they heard.

Mitchell, accusing Poindexter of "lying," said that the Navy rear admiral on 184 occasions in his testimony said he could not remember aspects of the affair, even though at other points he recalled events in fine detail.

Committee members said that the hearings showed that the strategy of creating a deniability mechanism failed. Others said setting North or Poindexter up as the "fall guy" -- the individual who would take the blame if the operation became public -- also failed to limit damage done by revelation of the operation.

Mitchell and Rep. Dick Cheney (R-Wyo.) said they also mistrusted Attorney General Edwin Meese III's testimony that Casey did not know of the affair, asserting that Poindexter could not have been the only high-ranking official to know of the plan.

"In light of the fact that the secretary of state and defense both opposed the Iran policy . . . probably it took someone as powerful and influential as Bill Casey who weighed in on the other side," Cheney told reporters. "It would appear, based on the evidence we've seen, that while in {one} role he was managing the CIA, in another role he had an office at the White House where he clearly was a close adviser to the president."

Democrats and Republicans on the committees said they hoped Reagan could recover from whatever political damage was done by the investigations and move on to pressing problems that need the government's undistracted attention.

"He has an important meeting with {Soviet} leader Mikhail Gorbachev," said Inouye. "We have many legislative issues to contend with, so if he has been injured by this unfortunate chapter, I hope his wounds will heal fast."Staff researcher Michelle Hall contributed to this report.