Leaders of the House and Senate committees investigating the Iran-contra affair have agreed to make several concessions to former White House aide Oliver L. North Jr. in hopes of ensuring that North will testify publicly in congressional hearings next month, informed sources said yesterday.

The proposed concessions, which emerged from a 90-minute meeting between North's attorney, Brendan V. Sullivan Jr., and the committee chairmen and vice chairmen, include an agreement that North can testify publicly ahead of his one-time boss, former national security adviser John M. Poindexter, probably beginning July 7.

The leaders also agreed that North's testimony will not last more than 25 to 30 hours, and that the preliminary, closed-door questioning that precedes it will entail only limited interrogation under oath. This private, sworn testimony, it was understood, will mainly deal with North's knowledge of what President Reagan and other top officials knew about the key aspects of the Iran-contra affair.

The sources did not predict whether Sullivan would agree to the leaders' proposals. Sullivan declined to comment.

The congressional leaders reportedly rejected another request from Sullivan that the committees would guarantee North would not be called back for additional questioning at a later date.

The leadership's proposals will be presented to the full select committees today, and could cause some heated discussion, particularly among House Democrats, who have grown increasingly restive over the possibility that a precedent may be created for protracted negotiations with Congress by witnesses subpoenaed in investigations.

North was originally subpoenaed to appear with his records last Wednesday, to be followed by closed-door questioning Thursday. But Sullivan objected that the closed sessions and the scope of the subpoena went beyond the grant of limited immunity from prosecution under which North was appearing. On Thursday, the committees sent North a new subpoena ordering him to show up today with his documents. This deadline has now been moved to Wednesday. But sources said that under the new proposal, North's documents would not be provided until June 30.

Until last week, the plan was to have North testify after July 16, following the public appearance of Poindexter. At the request of Independent Counsel Lawrence E. Walsh, the committees have attempted to delay North's testimony as long as possible to allow Walsh to accumulate evidence against him. North's testimony cannot be used in court against him under his grant of immunity.

The agreement for North to testify prior to Poindexter, as requested by Sullivan, is significant because it means the committees will question North without being able to refer to the public testimony of his former boss.

However, senior committee members point out that the sworn, private depositions of Poindexter will still be available to North's interrogators.

North's legal maneuvering has caused the most serious divisions to date in what has been an unusually unified House-Senate inquiry.

Last month, some members of the House committee objected to granting immunity from prosecution to North. Some House Democrats now argue that to make concessions to a witness under subpoena is "unseemly," according to one member.

Another source said that North and his attorney are "leading Congress around by the nose." This source suggested, for example, that North is exploiting the fact that the select committees have agreed to wind up public hearings in the first week of August.

Were North to refuse to testify, or to raise further conditions, the ensuing criminal contempt proceedings against him could last well beyond that date. Even if he were convicted, the maximum penalty for criminal contempt of Congress is only one year in jail and a maximum $1,000 fine.

One member declared with some bitterness yesterday that "right now, one outcome of this investigation ought to be {for Congress} to increase the penalties for contempt because the present penalties are hardly strong enough to back up a demand for a person to testify and produce papers."

Hearings will resume at 9 a.m. today in the caucus room of the Senate Russell Office Building. Witnesses expected to testify are Glenn A. Robinette, a security contractor and former CIA employe who arranged for a security system at North's Great Falls home; Henry H. Gaffney, director of plans for the Defense Security Assistance Agency, and Noel Koch, former principal deputy assistant secretary of defense for international security affairs.