Working since last December, a small State Department group has put together a massive, top-secret report that describes four decades of U.S. involvement in Iran, The Washington Post has learned. The report consists of a 500-page overview and about 60,000 pages of government documents. The overview includes a 100-page summary and shorter sections on various events and issues.

It is said to contain no bombshells, but rather to acknowledge in official from U.S. conduct that has never been admitted in the past and that has, in some instances, been denied.

The documents, however, are said to include sensitive private communications between U.S. officials and highly classified government reports which could, like the Pentagon Papers of a decade ago, cause a furor if made public.

The report was compiled on personal instructions from President Carter, who, according to one source familiar with the project, wanted to know "what was on the record in preparation for internation tribunal," with the idea of making much of it public.

An inquiry by such a tribunal was envisioned at the time as a likely step in the negotiated release of the Americans held hostage in Iran.

Nothing came of it then, but in the last few days, statements emanating from Iran suggest once again that an inquiry into U.S. activity in Iran over the years wil be a key requirement for release of the hostages.

The U.S. government has agreed to cooperate, providing the hostages are released at the outset of such an inquiry. The Iran papers, if the report may be called that (it has been given no name by the government), would be used to prepare much of th U.S. response to any inquiry today. The government, however, now has no intention of making any of the report public.

Iranian leaders have demanded occasionally that the United States admit "guilt" and apologize for its conduct toward Iran. One high-ranking administration official told The Post that the study does not constitute an apology but is "a kind of full laying out of ourrelations with the shah."

Several sources familiar with the report told The Post that if its supporting documents were aired, every president from Franklin D. Roosevelt through Carter would be subject to substantial critism.

At the same time, however, the sources said, Carter and national security adviser Zibigniew Brezinski have refused researchers' requests for specific material describing their own actions during the prolonged Iranian crises.

One source said the report as it now stands would give critics of U.S. policy a field day, and would show Carter in a very unflattering light despite his failure to provide material.

Another said, however, that if what the Iranians want is an admission of U.S. guilt for activities in Iran, the report "is not what they want. It will make them very mad."

In response to an inquiry from The Post yesterday, the White House had no comment.

State Department spokesman John Trattner said last night, "This is not the Pentagon Papers . . . there is no 'study' as such, only a collection of documents and factual summaries on past U.S. relations with Iran. This compilation did not focus on 'U.S. misdeeds,' and no conclusions or judgements wre derived. Essentially, it is an inventory and nothing more."

The report is said to be so secret that only 20 people in government know of its existence. A great many people contributed to portions of it but did not know they were helping put together such a history, one source told the Post.

Only two copies of the report and the accompanying documents are said to exist, with one of them being held by Brezinski, who now is in control of the study. The other copy is said to be locked in a place where only two State Department officials have access to it, apparently to prevent leaks or, according to some sources, to keep Brzezinski from making changes or destroying material.

The study grew out of a request by Carter for a white paper -- a document that could be made out of a request by Carter for a white paper -- a document that could be made public and would justify the U.S. role in Iran for the past 25 years. Four members of the State Department's policy planning and intelligence offices were assigned to conduct it.

Anthony Lake, the director of the policy planning staff, was put in overall charge. The operational head of the study was Phillip Stoddard, who worked on it while on leave from the intelligence office at State.

Almost immediately, the study team decided to push the period covered by the report back from 1953, when the CIA assisted in a coup that restored the shah of Iran to power, to 1941, when the United States first became deeply involved in Iran.

The team also concluded that the material should not ne prepared as a white paper to be made public. "If the Carter administration got into drawing judgements about the policy of each administration, they would have ended up condemning elements of every policy, including their own," one source said.

Initially, Carter resisted the change to a secret study, saying, according to one source, that "we have nothing to be afraid of." He was persuaded to go along by former secretary of state Cyrus R. Vance, who reportedly told him that a white paper would needlessly antagonize officials of past administrations and reflect poorly on the president during his reelection campaign.

Files from the Defence Department and State Department were immediately made available to the researchers, virtually in their entirety. The CIA provided access to information from its files, but would not part with the files.

The withheld CIA material was pieced together through discussions with CIA staff and through State Department files pertaining to CIA activity.

One source told the Post that material supplied by Vance and by Defense Secretary Harold Brown will reflect unfavorably on them. The report, for example, is said to make reference to an admission by Vance that he paid insufficient attention to the mounting Iranian crisis because he was distracted by arms limitation talks with the Soviet Union and the Camp David Middle East peace negotiations.

Brown, according to the source, is said to have concurred in recommendations urging the shah to crack down on dissent in Iran by turning the military there loose on demonstrators.

Sources said that Carter and Brzezinski refused to part with some information regarding their own roles on the grounds that presidential communications traditionally are not included in such studies. Further, they are said to have maintained, possible public disclosure of such information could endanger the lives of the 52 American hostages.

One source said that, in his view, however, Carter and Brzezinski seemed to be acting less in regard for tradition or safety of the hostages than out of a desire to keep criticism from being directed at them. Another source quoted Vance as saying that Carter would wait until after the election before deciding whether to provide document that have been requested from him.

Similar papers of former presidents Richard M. Nixon and Gerald R. Ford also have been withheld. In addition, apart from what was left behind at the State Department and the National Security Council, many of the relevant files of Henry A. Kissinger, former national security adviser and secretary of state, are said to be missing from the report.

In essence, one source said, the Iran papers contain "no shocking, previously unsuspected skulduggery," but rather are replete with material that confirms "everyone's preconceived notions."

For example, that source said, the study does not discuss in any detail the role of U.S. oil companies and arms suppliers in the formulation of policy toward Iran, but it is said to show clearly that government policy was responsive to the wishes of those special interests.

Sharply contrasting pictures emerge when people familiar with the report attempt to characterize it. One person said the story told by the Iran papers "is one of the United States repeatedly having its chain jerked by a guy [the shah] who every ambassador found impossible to deal with. Ultimately, it was capitulation by thoses who knew better."

That source said the report shows that at every step of the developing relationship, serious objections were raised by top foreign policy advisers to providing additional aid to the shah, but that each objection was gradually overcome by the sheer force of the shah's will. Each time, it was deemed essential to the security of the region to give in to the shah.

Another source, however, said the papers would reinforce the perception that the shah was, if not a U.S. puppet, at least a most willing and responsive partner in American policy.

The report, none of which has been seen by the Post, is said to have a 100-page section summarizing relations with Iran from 1941 to the early 1970s. It is said to have 15 to 20 shorter sections focusing on the CIA's assistance to the shah in 1953, on U.S. human rights policy in recent years in regard to Iran, on U.S. military influence over the Iranian military, on the effect since 1972 of U.S. arms sales to Iran, on U.S. contact with members of the Iranian military who were planning a coup during the period of the shah's collapse, and on the decision last October to admit the shah into the United States.

One reason the report has been given no name, the Post was told, was to make it more difficult for Congress to demand it or for media to seek access to it through the Freedom of Information Act. For the same reason, one source said, a decision was made to keep the report in a perpetual "craft" stage so that there would be no "final" copy, thus making it exempt from FOI request.

According to State Department sources, should the Iranian government demand an American "apology" for the release of he hostages, a document would be drafted, taken from the Iran papers, "to eat a certain in accord with an Islamic tradition that allows one to atone for past actions without total humiliation.

One high-ranking State Department official said such a document could be considered an apology. No such document would be released until a firm agreement on release of the hostages was in hand, he said."We don't want to give them any more of what they want without anything in return."

According to administration officials, another major study on the U.S. role in Iran is being prepared by the Senate Intelligence Committee under the direction of staff director William G. Miller, a former Foreign Service officer stationed in Iran and a critic of U.S. relations with the shah. The Senate panel staff, however, has not had access to the sensitive documentation provided to the State Department study group.

"We did not withhold the information, so much as they did not ask for the right information," one State Department official said.

The expressed purpose of the Senate study is to assess how well intelligence information was used in formulating U.S policy toward Iran.