AT ONE OF their earliest executive sessions last fall, each member of the House Assassinations Committee was handed for eyes-only inspection a black looseleaf binder about the murders of President Kennedy and Martin Luther King Jr.

In keeping with the "sensitivity" of the information it contained, all the members were required to surrender their copies at the end of the meeting.

The press and public have yet to get a glimpse of the secret briefing book. But it was all old stuff to Mark Lane, the quarrelsome critic of the Warren Commission and other assassination investigations. He had already been shown a copy in the offices of the then-chief counsel of the committee, Richard A. Sprague.

"I thought it was a remarkable job in a very short period of time," says Lane. He suggested this may have been partly due to the fact that he himself had given a number of briefings in the course of the committee staff's preparation of the report, a so-called "threshold analysis" of the two murders.

Mark Lane is riding high in the 14th year of his endeavors as entrepreneur, circuit rider and self-appointed revisionists of the Kennedy and King assassinations. His headquarters sits atop Capitol Hill. He was the genial host at a press conference here earlier this month to kick off the promotion for his latest enterprise, with Dick Gregory. It is a book entitled "Code Name 'Zorro': the Murder of Martin Luther King Jr.," and its publishers have tentatively earmarked $50,000 to turn it into "a number one best seller immediately."

Dozens of reporters from such varied organizations as Agence France Presse, the Nashville Tennessean, the Cleveland Plain Dealer and Jornal do Brasil showed up. Lane, just turned 50, held forth with the confidence of a veteran pitchman, presiding over a mockup of the murder scene, labeling the FBI as "prime suspects" in the killing, and finally finishing to a small burst of applause from student admirers in the audience. Lectures, Books and Bumper Stickers.

THESE DAYS Lane operates from a somewhat cluttered, four-story townhouse in the toniest area of Capitol Hill, just across the street from the Supreme Court. His standard of living is modest, he says: a 1968 Volkswagen, three suits and a $500 savings account.

He gets $1,000 to $1,750 for his lecture performances. His Citizens Commission of Inquiry sells copies of his 11-year-old book, "Rush to Judgment," at $5.95 each, prints of the Zapruder film of the JFK assassination at $25 a crack and, for the budget-minded, bumper stickers and buttons asking "Who Killed Kennedy? Ask Congress" for half a dollar apiece.

Lane says he gives all his lecture proceeds, $70,000 in the past two years, to the Citizens Commission, an organization he controls.

A glib showman who can serve up a beguiling blend of fact and fiction, Lane moved to Washington in January of 1975 "following the Watergate disclosures - and when it became plain to me, for the first time, that Congress might be convinced to conduct an investigation of the Kennedy assassination."

He established the Citizens Commission, "a non-profit organization devoted to a congressional reopening" of the issue, bought what he describes as "an old boarding-house" at 102 2d St. NE as a headquarters and residence and put together a staff or dedicated college students to watch the store while he galloped about the country to assailt the Warren Commission, the FBI, the CIA and others, drawing bigger and bigger crowds as the months wore on.

The preaching does not come free. His booking agent charges "as low as $1,000 for a straight lecture" to $1,750 for "a three-day multimedia program," Lane says. The agent takes a third off the top and sends the rest to Washington.

By late 1975, in a climate of distrust fed by Watergate and subsequent disclosure of CIA and FBI misdeeds, Lane was averaging 10 lectures a week. Nearly 7,000 turned out to hear him at Purdue University. At Northeast Louisiana University in Monroe, La., he outdrew Ronald Reagan, who had been there a few weeks earlier.

But by his account, Lane seems never to strike it rich. He counters suggestions to the contrary as a confection of the CIA, a line of inquiry suggested by the agency years ago in a memo entitled "Countering Criticisms of the Warren Report." Lane oftens responds to his critics in kind, accusing them of lying, ulterior motives and sometimes mental illness.

He also pleads poverty when asked how much he made off the film version of "Rush to Judgment."

Although the British Broadcasting Corp. once paid $40,000 for a single showing, reportedly the biggest such fee ever paid in Britain, Lane says he got "not a farthing" out of that telecast or any other showing. He maintains that co-producer Emile de Antonio "seized control of the film" before it started making any money.

Lane and writer Donald Freed also wrote the script of another film, "Executive Action," a fictionalized version of the JFK assassination which grossed $15 million, but the two have been quoted as saying they got "very little" for their efforts. Building Pressure on Congress

BEFORE THE Kennedy assassination in 1963, Lane was a little-known New York lawyer and one-term member of the New York State Assembly. After the murder, he wrote a quick "Brief for the Defense" in a New York weekly and sought to establish himself before the Warren Commission, first as counsel for the dead Lee Harvey Oswald and then for Oswald's mother, Marguerite.

Then he hit the lecture circuit and in 1966 came out with his first book, "Rush to Judgment." Although one critic estimated that "nowhere near a tenth of Lane's relevant assertions and implications would stand up to careful scrutiny," more than a million paperback copies of the book were sold and more than 125,000 in hardcover.

Today Lane has not only helped turn the assassinations, first of Kennedy and then of King, into a thriving cottage industry; he has also become something of an oracular figure for the congressional panel investigating the two murders.

It is not secret that Lane was instrumental in lobbying for the creation of the select House committee as well as the choice of Richard A. Sprague as its first chief counsel.

According to Lane, as many as 180 Citizens Commission chapters were set up, many of them consisting of just a college professor and/or a couple of students, but active enough to stage showings of the Zapruder film and other meetings that would draw hundreds.

"We helped generate more than three-quarters of a million telegrams and letters to members of Congress," Lane declares. Some, such as Rep. Joe Moakley (D-Mass.), a member of the crucial House Rules Committee, got special attention. According to Lane, Moakley said he'd been advised by Tip O'Neill (D-Mass.), then House majority leader, not to get involved, but Moakley changed his mind after a committee of Citizens Commission members in his district collected 2,500 signatures in a single day. At another point, Lane flew to San Francisco to orchestrate pressure against Rep. Phillip Burton (D-Calif.), who had been reported in the press as saying he would try to keep the issue from even coming up for a vote in the House.

But even all this failed to produce the requisite momentum until Lane hooked up with a friend, television producer Abby Mann, on a trip to Memphis last summer to gather material for a documentary on King. What they came up with - the unexplained removal, shortly before King was shot, of one of two black detectives assigned to keep King under surveillance and the abrupt transfer of two black firemen from the firehouse across the street from King's motel - was nothing new, but they transmitted it as startling new information to D.C. Del. Water E. Fauntroy and the rest of the Congressional Black Caucus.

The House leadership relented soon thereafter. The committee was created last Sept. 17 by a vote of 280 to 65. Defending the Committee

LANE HAS ALSO taken on a self-appointed role as defender of the committee against press attacks. His influence on the atmospherics of the investigation has been considerable. He has been, for example, at least the harbinger of what some consider "a new kind of McCarthyism" that has come to surround the committee. Any criticisms, any unfavorable publicity, indeed any vigorous insistence that the two assassinations may already have been solved can then be ascribed to secret paid-up membership in, or fellow-traveling with, the CIA or the FBI or both.

George McMillan, author of "The Marking of an Assassin," says he felt the sting as long ago as last October upon publication of his book about James Earl Ray as a poor white racist who reportedly decided, months in advance, to "kill that nigger King." McMillan was invited to appear on NBC-TV's "Today" show last Oct. 26 with Fauntroy, chairman of the subcommittee investigating the King murder.

The two men had never met and, McMillan says, "I never dreamed Fauntroy would attack me on that show that morning. But he came in with some 3-by-5 cards and, from the moment he came in, he just studied them. They had the same things on them that Lane was saying. One was led to think that Lane even prepared them."

McMIllan was caught by surprise. "I'd been accused of being a Communist years ago," he says. "That wasn't so, but I had been quite active in the civil rights movement, I'd been a writer in residence at black colleges in Atlanta and to be treated [on the 'Today' show] as a friend of the FBI was quite uncongenial for me . . . I didn't do too well on that show. I never even got a chance to explain what my book was about . . . This is just a new kind of McCarthyism."

As a matter of fact, Fauntroy did get his tips from Lane, on a visit to Lane's headquarters-home on Capitol Hill the night before the show. Fauntroy said he went to see Dick Gregory, who was staying with Lane on a visit to Washington, and the talk turned to the "Today" show.

"It is true I made a number of suggestions [to Fauntroy] about what could be raised," Lane says!

Other examples of Lane's use of the offense as defense:

When Rep. Don Edwards (D-Calif.), as chairman of the House Subcommittee on Constitutional Rights, protested against the intrusive nature of the hidden recording devices, voice stress analyzers and other gadgets proposed for the investigation by chief counsel Sprague, Lane began denouncing him on the lecture circuit and noting darkly that Edwards was a former agent of the FBI. Up to that point, Edwards had been mentioned favorably in Citizens Commission newsletters for holding congressional hearings in 1975 on the FBI's long-secret destruction of a threatening note from Lee Harvey Oswald.

When news articles began to appear, highlighting the House committee's budgetary and civil-liberties problems and raising questions about Sprague, Lane responded by attacking certain "segments" of the press. On one radio show in late January, he asserted that the House committee was facing "one of the most effective campaigns ever waged by what they call the 'intelligence community'" and then went on to complain of the press coverage by "Jeremiah O'Leary of The Washington Star . . . David Burnham of The New York Times . . . and . . . George Lardner of The Washington Post."

Once again, Fauntroy picked up the theme, warning in a television interview April 24 that the Assassinations Committee might investigate "a few" of the reporters assigned to cover the inquiry and charging that some of them might be CIA agents. Asked later whether he had talked with Lane about the subject before making his charges, Fauntroy said that he "probably" had.

As for his future rule in the House investigation, Lane said he intends to keep supplying it with whatever relevant information he comes across - although there are indications that he may now know as much as he once said he did.

Writing in a Danish newspaper in 1967, Lane said in a signed article, "I know who fired the fatal shot at President Kennedy." Three years later, in another article, this time in the Los Angeles Free Press, he assured readers that the "CIA Killed JFK."

Which is all very strange. Last Sept. 17, after the House had voted that day to establish the Assassinations Committee, someone claiming to be Mark Lane declared firmly on WWDC here that "I don't know who killed Martin Luther King." And then, just last month, on WMCA in New York, someone again purporting to be Mark Lane assured listeners there: "I've never said that the FBI and the CIA were involved in the assassination. I still have not said that . . ."

Could there be a false Mark Lane?