The last time Lt. Col. Oliver L. North appeared before Congress, he was hailed by one member as "truly a great American." Another legislator went so far as to substitute North's name in a Rudyard Kipling poem while reading aloud a passage honoring soldiers who fight for their country.

Smartly dressed in his Marine uniform with six rows of ribbons, North seemed to embody the all-American patriot even as he told the House Foreign Affairs Committee last Dec. 9 that, on the advice of his attorney, he had decided to remain silent. Biting his lip and speaking in a quavering voice, North said: "I don't think there's another person in America that wants to tell this story as much as I do, sir."

Today, North will finally get a chance to tell his story as the Senate and House Iran-contra panels begin four days of intense questioning of the man who has become the most intriguing figure in the biggest political scandal since Watergate.

But in returning to face Congress at 9 a.m. today under a grant of limited immunity, the fired National Security Council aide now faces an uphill battle.

In the past seven months, assorted disclosures in the press, the Tower review board report and many weeks of congressional testimony this summer have revealed a guileful, conniving side to North that contrasts with the straight-arrow image of his last appearance. What has emerged is a portrait of a North who is more complex and elusive than was evident last winter.

"I still think he brought creativity and admirable qualities to such incidents as Grenada, Libya and the Achille Lauro," said Rep. Bill McCollum (R-Fla.) of the House panel, referring to North's role in helping direct U.S. military actions. But McCollum added: "He's got a dark side as well."

For congressional investigators, who have billed North as one of the two or three witnesses most crucial to solving the scandal's remaining mysteries, this "dark side" has created a perplexing problem: Can North be believed?

Rep. Michael DeWine (R-Ohio), an administration supporter on the panels who recently called North a "liar," said last week that "because he's lied once doesn't necessarily mean that he can't be believed. It does mean that when you judge his credibility, you have to take into account all the known facts."

North's image was clearly hurt by recent testimony that after the FBI began investigating him last December, North concocted an elaborate alibi to conceal the fact that others had purchased a $13,900 security system for the North home in Great Falls, Va.

Glenn A. Robinette, a former Central Intelligence Agency official who said he arranged for the system at the request of retired major general Richard V. Secord, testified that North sent Robinette two fake letters that had been backdated. The letters apparently were intended to create the impression that time had passed between the writing of the first and the second, and that North was seeking a way to pay for a system that in fact already had been paid for by others. The more formal first letter was addressed to "Mr. Robinette," while the second was informal and began "Dear Glenn."

In both letters, Robinette said, North alluded to a novel -- but fictitious -- proposal of paying for the system by making his home available for the "commercial endorsement" of Robinette's firm. Testimony also showed that someone went to the trouble of typing the second North letter on a machine on which the letter "e" purposely had been filed down, apparently to indicate that two different typewriters were used or that there had been an interlude between composition of the letters.

North's credibility also has been eroded by other revelations, according to congressional sources, including evidence that he cashed traveler's checks provided by a contra leader at grocery stores, a gas station, a tire shop and a women's hosiery outlet. Among other episodes cited are North's statements, now known to be false, to a House panel last August that he "knew of no specific military operations" planned by the Nicaraguan contras, and had limited his contact with the rebels to fostering a "viable, democratic political strategy."

"I think we've seen an evolution in the image of Col. North from a pure patriot to someone who engages in petty activities," said Sen. William S. Cohen (R-Maine), a member of the Senate panel.

North's supporters said they are confident he will have an explanation for his actions. For example, one friend said, North may assert that he reimbursed all monies collected after cashing the traveler's checks.

Although some have depicted North as a "loose cannon" setting his own foreign policy course for the administration, congressional investigators describe him as habitually given to keeping his superiors informed through memos, directives and other methods.

For example, after authorizing a plane carrying arms to proceed from Tel Aviv to Tehran in May 1986, North awakened former national security adviser Robert C. McFarlane to advise him of the move and dutifully rescinded the order when McFarlane angrily objected.

Even the secret diversion of funds to aid the contras from the Iran arms sales, which North described in his memo of April 1986, was sent for approval to national security adviser John M. Poindexter. Whether it went higher, to the president, remains uncertain; President Reagan has said he was unaware of the diversion.

In evaluating North's testimony this week, investigators also will consider accounts from several witnesses who said that North repeatedly told colleagues he was prepared to be the "fall guy" in the scandal.

Robert W. Owen, North's self-described courier to the contras, testified that North repeated his vow to take the rap last Nov. 25, the day he was fired from the NSC.

"Bill Casey {then CIA director} knows it, and others know it, and I'm ready to take that responsibility," Owen quoted North as saying.

Other major issues likely to come up this week include:

The role of Reagan and other officials in the secret effort to raise funds and provide other assistance to the contras during a two-year congressional ban on most U.S. military aid to the rebels.

North's reliance on a network of businessmen and former CIA agents led by Secord. The network, which North used for both the contra and Iran operations, had previously been linked to a federal investigation of former CIA operative Edwin P. Wilson, who is serving a prison sentence for providing training and explosives to the Libyan government of Moammar Gadhafi.

North's knowledge of whether Secord and other members of his operation profited from the Iran arms sales. Records made public by the committees show that middlemen working with Secord funneled $8 million into Swiss bank accounts while steering only about $4 million to help the contras.

North's understanding of whether the Iran arms profits belonged to the U.S. government or, as Secord asserted, to the "enterprise," Secord's name for the network of dummy corporations and Swiss bank accounts set up for the Iran and contra initiatives.

North's role in a November 1985 shipment by Israel of American-made missiles to Iran. This shipment and one in September 1985 apparently violated the Arms Export Control Act, which governs the transfer of U.S. arms to third countries.

His knowledge of whether there was an orchestrated cover-up by administration officials to hide the role of the president and other senior officials when the scandal became public last November. North is likely to be questioned about his contact with the much-criticized Justice Department inquiry conducted by Attorney General Edwin Meese III. The committees have heard evidence that North directed the shredding and altering of NSC documents around the time of Meese's inquiry.

North also is expected this week to either contradict or corroborate the statements of several former and current high-level administration figures, including former national security advisers McFarlane and Poindexter, Secretary of State George P. Shultz, Assistant Secretary of State Elliott Abrams, Vice President Bush, Meese and Casey.

For example, Abrams testified adamantly under oath that he was not involved in North's private contra support network and did not know of North's role in an air resupply operation for the rebels.

North helped organize and direct the private airlift with the knowledge of then-U.S. ambassador to Costa Rica Lewis A. Tambs, who worked under Abrams, and with help from the CIA station chief in Costa Rica.

North also will likely be questioned about whether Bush knew of the air resupply operation and other private efforts to support the contras. Bush was linked to reports on the contra air resupply operation last year through his national security adviser, Donald P. Gregg, who is an old friend of Felix I. Rodriguez, a former CIA operative recruited by North to help the private airlift.

Bush, who has acknowledged meeting Rodriguez three times, has denied discussing the contras with him or knowing of the air resupply operation.

North helped raise several million dollars for the contras, largely through his collaboration with private conservative fund-raiser Carl R. (Spitz) Channell, according to several witnesses. Using the prestige of the president and the White House, North and Channell successfully operated, as one Republican senator said, as a "one-two punch" with wealthy conservative donors.

For example, North's vivid descriptions of the communist threat in Central America would be promptly followed with a pitch for money by Channell, who offered to set up private meetings with Reagan for anyone contributing $300,000 or more, according to testimony.

Since the hearings began, the White House has revised its statements about Reagan's knowledge of contra fund-raising and other private efforts to support the rebels.

"I had no detailed knowledge" about efforts to raise military aid for the contras, Reagan said two days before the Iran-contra hearings started.

Ten days after the hearings began, Reagan reversed himself and said he was "very definitely involved in the decisions about support to the freedom fighters."

"I was kept briefed on that," Reagan said, adding that it was "my idea to begin with."

The precise nature of North's relationship with Reagan will receive close attention.

Fawn Hall, North's NSC secretary, testified that North never received any telephone calls from the president and that she was unaware of any "one-on-one" meetings with Reagan, although she recalled that North once said he had been "at the residence."

The day North was fired, he received a phone call from Reagan, who later publicly hailed North as a "national hero" in a press interview.

Secord, who was with North, testified about the call.

"I didn't realize it was the president until I saw {North} stand up at attention as a good Marine, you know. And, he said, 'Yes, Mr. President. Yes, Mr. President. Thank you very much, Mr. President,' " Secord said.

"I'm just sorry it had to end this way," North was quoted as saying. "I was just trying to serve you the best way I knew how, Mr. President."

Staff researcher Michelle Hall contributed to this report.