Only minutes after Lt. Col. Oliver L. North left the witness chair yesterday, his former boss and longtime friend, Robert C. McFarlane, returned to the same seat to contradict numerous aspects of his former subordinate's sworn testimony, including North's fundamental defense that his superiors had authorized everything he did as a National Security Council aide.

In a late-afternoon session whose low-key mood was a sudden change from the electric tension of North's six days as a witness, McFarlane quietly attempted to weaken the case his former protege and close friend had made. He challenged North on the ground where the Marine officer had made his best impression before the Iran-contra committees -- his repeated insistence that he always acted with proper authorization. McFarlane's contradictions of North on that point met with a skeptical response from the committees.

"I did not authorize many of his {North's} activities," McFarlane said.

There were cases, McFarlane added, when North "went over the line from advice to . . . an operational role. And that was not authorized."

McFarlane also questioned one of North's basic and oft-repeated explanations for his controversial activities: that they were known and approved by the late CIA Director William J. Casey. "If Bill Casey was going to do all the things he's been charged with, why didn't he tell me?" McFarlane asked almost plaintively, noting that the two met weekly.

In other developments yesterday:North, before the end of his testimony, revealed that he had been offered a $1 million bribe by Iranian middleman Manucher Ghorbanifar during a bathroom conversation in a London hotel in January 1986. North described the offer as an "enticement" to facilitate a problematic U.S.-Iran arms transaction then in doubt. North said he turned down the bribe but picked up the "far better idea" Ghorbanifar suggested immediately afterward in that bathroom conference -- to use proceeds of the arms sale to help the Nicaraguan rebels. North said that the Restricted Interagency Group (whose chairman was Assistant Secretary of State Elliott Abrams) had "widely discussed" clandestine military support for the contras, including the building of an airfield in Costa Rica to service planes dropping supplies to them, at a time when Congress had banned U.S. aid. Abrams steadfastly denied having such specific discussions with North when he testified. North, under questioning from Rep. Edward P. Boland (D-Mass.), said Casey told him not to take notes at their meetings, explaining why an "inveterate note taker" like North had nothing in his personal spiral notebooks about his meetings with the CIA director. "Put away the notebook -- if I couldn't remember it I don't belong in the business," North recalled Casey telling him. Rep. Louis Stokes (D-Ohio) brought out during his questioning of North that a British mercenary company run by David Walker was hired to carry out missions inside Nicaragua and was responsible for blowing up an arms depot in Managua on March 6, 1985. North said he personally had "no role" in the operation, but understood that Walker was paid either by the contras or retired major general Richard V. Secord, who ran "the enterprise" that resupplied the contras and helped deal arms to Iran at North's behest.

North said he was "quite certain that I sought the approval of my superiors" -- either McFarlane or the then-deputy at the NSC, John M. Poindexter -- for proposing that the rebels hire Walker. McFarlane said yesterday that he had heard Walker's name, but not in the context of the Nicaraguan operation. North said his close friend, CIA official Duane (Dewey) Clarridge, had "general knowledge" of the private, North-supervised air resupply operation run by Secord in Central America. He also said that he talked to Clarridge about his operation and "used his expertise." Congress had banned the Central Intelligence Agency from providing direct aid to the contras at the time.

McFarlane, looking weary and older than at his first committee appearance almost two months ago, returned to testify at his own request. In addition to defending his integrity, McFarlane was also attempting to resolve conflicts that could pose potential legal problems, since both McFarlane and North have testified under oath. North received from the panels limited immunity from prosecution, though this would not protect him from a charge of perjury. McFarlane had no immunity.

The committees' decision to allow McFarlane's appearance yesterday was the latest in a series of concessions the panels have made to witnesses. Earlier in the day, after a number of Republican members sharply demanded that North be permitted to give his famous slide show promoting support of the contras, Senate committee Chairman Daniel K. Inouye (D-Hawaii) announced that North would be permitted to deliver it without pictures.

For McFarlane, the committees waived the rule that no opening statement be permitted unless delivered to the panels 48 hours in advance. Moreover, in a move unprecedented in these hearings, the committees limited the questioning of McFarlane to members only. The move restricted the role of the staff attorneys who have done most of the examining of key witnesses.

McFarlane mostly repeated statements he had made to the committees in his initial testimony in May. His testimony left the committees with a problem, because many contradictions between it and North's version involve conversations in which only the two participated.

In effect, McFarlane's testimony painted North, whom he variously referred to as "Col. North" and "Ollie," as an independent operator who set up covert activities while informing McFarlane of what he was doing only in the most general way. On the other hand, McFarlane emphasized, he did not believe there was any "malice or deception" involved. He went on to suggest that North may have been keeping the facts from his boss in order to protect him -- just as North told the committees that he was willing to be the "fall guy" to protect the president.

When his chance came, Sen. Sam Nunn (D-Ga.) ticked off a series of points of conflict that he had already reviewed earlier in the day with North.

According to North, McFarlane approved the idea -- suggested by Casey -- of approaching Secord to help in what eventually became a private, covert operation selling and airlifting arms to the contras. McFarlane testified late yesterday that he did not recall learning of Secord's involvement until December 1985.

North testified that McFarlane knew of and approved his speech-making on behalf of the fund-raising efforts of Carl R. (Spitz) Channell. McFarlane said he did not know of North's association. He said, "The existence of the Channell group . . . came to my attention, when I suppose it did to everyone else here, a few months ago" -- when Channell and an associate pleaded guilty to conspiring to defraud the government in his fund-raising.

North and McFarlane also differed over which of them suggested altering 1985 documents pertaining to North's activities in support of the contras. McFarlane testified that in the summer of 1985 North had made suggestions for changes in the originals of certain documents, but McFarlane had refused, and was unaware that they were later altered until North testified to that last week.

North, on the other hand, told the committees that McFarlane had authorized him to "fix" the documents in mid-1985, but he did not get around to doing it until last November.

In the face of a growing record of testimony and documents to the contrary, McFarlane asserted once again yesterday that his only involvement in a highly controversial 1985 transfer of U.S.-made Hawk antiaircraft missiles from Israel to Iran was "two or three phone calls in Geneva," where he was attending the U.S.-Soviet summit.

"Those calls were the extent of my involvement with the entire enterprise," he insisted. That contradicted testimony by North and others, as well as cables that are now part of the hearings record.

Documents released yesterday by the committees provided insights into North's elaborate efforts to maintain a low public profile as he became the center of NSC activities that later exploded into the Iran-contra scandal. Some of his efforts to stay out of public view evidently involved The Washington Post.

Notations in his notebook for Aug. 8, 1985, refer to a Post article on "TWA." The paper carried no reference to the TWA hijacking that day, but it did print a story that, for the first time, named North as a pivotal figure in the administration's Central America policy, "particularly in nurturing connections among antigovernment rebels in Nicaragua, their conservative supporters in this country and the rest of the administration."

The Post pointed out that North worked in a "grey area of government policy" for McFarlane, who refused to let North be interviewed but said "he's not a rogue elephant," but rather "like a son of mine."

The same notebook entry remarks that "{Defense Secretary Caspar W.} Weinberger is at war w/W.P. -- won't work," and another entry says: "Dan Murphy {then Vice President Bush's chief of staff} -- call K. Graham," a reference to Katharine Graham, chairman of the board of The Washington Post Co.

Then-White House spokesman Larry Speakes had asked The Post to withhold North's name, and North himself called Managing Editor Leonard Downie Jr. with a similar appeal. After the article appeared, Downie began receiving phone calls in the early morning hours from a male caller who repeatedly complained about the appearance of North's name in the article.

Downie wrote a letter to North in March 1986, informing him that he had been receiving "harassing telephone calls by an individual using your name." Shortly afterward the calls stopped.

North himself raised this matter in July 1986 when FBI agents questioned him in connection with the activities of Jack Reynolds Terrell, a soldier of fortune who had trained contras in Honduras.

Staff researcher Michelle Hall contributed to this report.