Tucked away on the fifth floor of the U.S. Courthouse here are the headquarters -- and secret records -- of a little-known Washington institution set up eight years ago in the aftermath of the Watergate scandals.

It is a three-judge court that has no courtroom, no public docket. Motions submitted to it are kept under seal. Its rulings are labeled "Confidential." It has never held a public hearing.

It is called the Division for the Purpose of Appointing Independent Counsels (formerly Special Prosecutors), but there is no door with that title on it. Officially it is an arm of the U.S. Court of Appeals for the District of Columbia, but only one of its members, Senior Circuit Judge George E. MacKinnon, sits in Washington.

The others are Senior Circuit Judges Walter R. Mansfield of the Second Circuit Court of Appeals in New York and Lewis R. Morgan of the 11th Circuit in Atlanta.

They have a busy month ahead. Attorney General Edwin Meese III reportedly asked Monday that an independent counsel be appointed to pursue allegations that administration officials -- including some who now are Meese's colleagues at the Justice Department -- improperly withheld Environmental Protection Agency documents from Congress in the 1982-83 controversy over toxic waste cleanup efforts.

It is now up to the three judges to choose a lawyer to run the investigation and to spell out his mandate.

In the EPA case, Meese took weeks before deciding that a 1,284-page report from the House Judiciary Committee merited even a preliminary inquiry -- a delay that highlights changes in the law in recent years.

Under the original 1978 Ethics in Government Act setting up the special court, an independent investigation would have been almost certain. The law required appointment of a "special prosecutor" whenever the attorney general received "specific information" of violations by high-ranking officials and was unable to certify, after a preliminary investigation, that the matter was "so unsubstantiated that no further investigation or prosecution is warranted."

President Reagan's Justice Department didn't like the law and Congress developed its own misgivings. Critics said the law set too low a standard, requiring even plainly frivolous charges to be taken seriously. So in 1982, it was changed to give the attorney general the leeway to decide whether allegations of misconduct in high places were "sufficient to constitute grounds" to begin even a preliminary investigation.

In days ahead, the court also is likely to be mulling over a much more closely held matter: a secret ruling it issued last month that could be a fatal roadblock to the criminal trial of former labor secretary Raymond J. Donovan and nine other men in a New York state court this fall.

A copy of the March 26 order was obtained by The Washington Post. It demands that prosecutors in the Bronx explain why it would be a "miscarriage of justice" if they are not permitted to use, at trial, the same federal grand jury evidence they used in obtaining the Donovan indictment in September 1984.

The order seems to be an abrupt turnabout from the same court's rulings in 1982 and 1984, when prosecutors for Bronx District Attorney Mario Merola sought and obtained copies of federal grand jury testimony and other sealed evidence compiled by special prosecutor Leon Silverman in his 1982 investigation of alleged ties between Donovan and members of organized crime.

Silverman found "insufficient credible evidence" to sustain a prosecution of Donovan on any federal charge. Records from his probe were locked in a walk-in safe at the courthouse here. Around the same time, Merola's office began its own inquiries, spurred by several potentially related gangland murders in the Bronx.

The special court, then headed by the late Circuit Judge Roger Robb, authorized Merola's prosecutors to copy 70 items of evidence from the Silverman probe to carry out their grand jury investigation and "enforce the laws of the state of New York." Donovan and his codefendants were subsequently accused of fraud and grand larceny in connection with a $186 million New York City subway project awarded to Donovan's New Jersey construction company.

Following the indictment, Donovan's attorneys asked the special court to retrieve the Silverman records, on the grounds that they shouldn't have been disclosed, but the three-judge panel rejected the complaint and the Supreme Court refused to review it.

Since then, much of the evidence, including testimony and statements from most of the defendants, has been made public in prolonged pretrial maneuvering. Defense lawyers have characterized it as "essential" to the prosecution's case.

In a routine motion in February, Bronx prosecutors asked the special court to "authenticate" the documents so they could be admitted into evidence at trial, now set for Sept. 2.

The panel, now headed by MacKinnon, treated the motion as if it were a request for fresh disclosure. In a three-page ruling underlined "Confidential," it gave prosecutors until April 25 to answer more than 350 questions. Why is each of the 70 items needed to avoid "a miscarriage of justice"? How does the need for disclosure of each exceed "the need for continuous secrecy"? Just how is each item to be used at trial? As substantive evidence? To impeach a witness? Refresh a recollection? Test credibility?

All this and more was concluded with a sternly worded "no disclosure" edict and signed, for the court, by Clerk George A. Fisher of the U.S. Court of Appeals here. Not even the trial judge in the Bronx case, John P. Collins of the New York Supreme Court, is entitled to know what it contains.

An amiable but tight-lipped officer of the court, Fisher declined to say much about the court, much less its inner workings. The judges are appointed for two-year stints by Chief Justice Warren E. Burger. They sometimes meet here, sometimes in New York or elsewhere, depending on the locale of the allegations they're dealing with, but Fisher would not say where. He would not even say whether there have been any special prosecutors appointed under the 1978 law besides the four who are known.

"I am at liberty to discuss only what has been made public," Fisher said. The four whose appointments were publicly announced were Silverman, named in 1981; New York lawyer Arthur Christy, appointed in 1979 to check allegations about Jimmy Carter's White House aide Hamilton Jordan; New Orleans lawyer Gerald Gallinghouse, named in 1980 to deal with allegations about Carter campaign manager Tim Kraft, and Washington lawyer Jacob Stein, appointed in 1984 to conduct a wide-ranging investigation of then-White House counselor Meese. None of the inquiries turned up sufficient evidence for criminal charges.

The court may have appointed other prosecutors to probe allegations concerning high-ranking government officials: Under the law, their identities and jurisdictions need not be made public unless the attorney general or the court decides that disclosure "would be in the best interests of justice" or unless criminal charges are ultimately filed.

"My lips are sealed except to what the court specifically releases," Fisher said. Asked why the March 26 order in the Donovan case needed to be secret when so much of the evidence at issue has already been made public, he said he would not "admit or deny" that such an order existed.