Fired White House aide Oliver L. North was questioned behind closed doors for nearly two hours yesterday by congressional investigators regarding President Reagan's knowledge of the diversion of funds from the U.S.-Iran arms sales to support the Nicaraguan rebels.

When last questioned seven months ago about what Reagan knew, North told Attorney General Edwin Meese III that he had never discussed the diversion with the president and did not believe Reagan ever received a memo that North had drafted regarding the funneling of funds to the contras, according to notes of that session shortly before the Marine lieutenant colonel was fired.

Yesterday, appearing under a grant of limited immunity from prosecution and wearing his Marine uniform, North told reporters on his way into the basement hearing room in the Rayburn House Office Building that he planned to tell "the truth" to congressional investigators. His only other response to reporters' questions was, "Have a nice day."

North was officially granted immunity Tuesday by the House and Senate committees investigating the Iran-contra affair and thus is protected from being prosecuted for anything he says to the committee, unless the testimony is proven false.

The interrogation lasted one hour and 45 minutes, according to a congressional source. Under an unusual agreement hammered out last week between North's lawyer and the committees, the questioning was limited to the narrow issue of North's knowledge of the president's involvement. In return for the committees' limited closed-door questioning, North guaranteed he would appear for public questioning beginning next Tuesday.

The chief counsels of the two committees, Arthur L. Liman and John W. Nields Jr., explored not only North's direct exchanges with Reagan on the diversion issue, but also what he had been told by other individuals, sources said. These would include North's former boss, Rear Adm. John M. Poindexter, whom the committee has already questioned for almost 15 hours behind closed doors.

During the initial, "off the record" questioning by the two counsels, North was not under oath, according to the agreement with his lawyer. He then was sworn in for a formal executive session of the committees during which a transcript was made. The questions were "broader than expected," one source said.

The committees plan to keep the testimony secret and make no comment about it until the public session with North begins next week.

North, who was fired Nov. 25 from the National Security Council staff at the time the diversion was announced, has since become the central figure in the Iran-contra scandal. On the day before his firing, North's lawyer at the time, Thomas Green, told a Justice Department official that North "wants to step forward and take the spears in his own chest."

North has previously discussed the diversion on several occasions with individuals involved in various aspects of the Iran-contra affair, according to documents and testimony introduced in Congress and presented by the Tower review board. But before today's closed-door hearing, he was reportedly questioned only once on the matter.

In a nearly four-hour session last Nov. 23 in the attorney general's office, North was quizzed by Meese, Assistant Attorney General William Bradford Reynolds, Meese aide John Richardson and Assistant Attorney General Charles J. Cooper.

Cooper's testimony last week to the congressional committees and the report of the Senate Intelligence Committee released last February provide the best rundown to date of what North said at that time. After being shown a copy of the April 1986 draft memo found in his files that contained a proposal to divert $12 million from an upcoming sale of arms to Iran, North told Meese that he:

Never talked directly to the president about the diversion.

Did not believe the president received the memo or one like it.

Did not believe the president had been briefed on the diversion plan.

Did not know if the plan was approved, but "didn't think so."

That last statement, described by Cooper from notes taken by his colleagues at the session, runs counter to what former national security adviser Robert C. McFarlane testified that North had told him.

According to McFarlane, on three occasions North told him that he would not have undertaken such a step if it had not been authorized. McFarlane told the investigating committees that North told him just hours before he went to see the attorney general that "you know it was {authorized}. You know I wouldn't do anything without approval."

McFarlane testified that he understood North's approval came from Poindexter, then-CIA Director William J. Casey and "the other principals who would have to be party to that kind of decision."

During his appearance before the committees last week, Cooper was not asked whether North had been questioned by Meese about why the diversion had taken place if it had not been authorized.