The court is so secretive that it deliberates in a vault-like room on the top floor of the Justice Department building. It never makes known the reasons for its decisions, and public inquiry into its operation has been limited to one academic study and one congressional hearing.

Yet the cases that it deals with are among the most sensational in the United States, and the court must rule on the most controversial elements of those cases.

The Foreign Intelligence Surveillance Court of seven U.S. District Court judges decides whether to allow federal electronic surveillance of persons or offices in the United States suspected of espionage or international terrorism.

Created after the disclosures a decade ago that U.S. intelligence agencies had carried on a variety of illegal surveillances under the guise of national security, the court was designed to ensure constitutional safeguards in espionage and terrorism investigations.

The few in Congress who have monitored the court's operations said the court has struck an effective balance between national security and individual rights. But civil liberties advocates are not so certain.

They contend that because the requests submitted to the court concern national security, the judges will instinctively favor the government's side.

In fact, the court has not turned down any of the government's 3,195 requests for electronic surveillance since it was established in 1979 under the Foreign Intelligence Security Act.

"Either the act is working perfectly, or it really isn't working very well at all," said Rep. Robert W. Kastenmeier (D-Wis.), chairman of a House judiciary subcommittee that held oversight hearings on the court in 1983.

Because the court's decisions to allow electronic surveillance are kept secret unless an indictment is produced, the targets of most of the requests are unknown.

But in practically every one of the recent spate of espionage cases, an electronic surveillance was approved by the court. For example, in October 1985, six weeks before Federal Bureau of Investigation agents arrested Ronald W. Pelton on charges of selling top-secret information from the National Security Agency to the Soviets, the court approved tapping of telephones at six locations that Pelton was known to visit. Pelton was convicted last month.

When the target is a U.S. citizen or a legal alien, the act requires that the government show there is probable cause to believe the person is involved in intelligence activity that "may involve" a criminal violation and that is being conducted for a foreign power.

That standard of proof is measurably less than that required of investigators seeking to conduct electronic surveillance in nonintelligence cases.

For instance, in cases involving organized crime, political corruption or racketeering, the government must show that the person to be tapped "is committing, has committed or is about to commit" a criminal offense.

"My concern is that the government might take advantage of the criteria difference, and contend it involves broadly a national security matter. In fact, if you looked at it closely, you'd see it's a straight criminal case," said Herman Schwartz, an American University law professor who is a specialist in national security law.

But since intelligence court decisions are made public so rarely, "we have little but trust to go on to tell us that things are not being abused," Schwartz said.

The Senate and House Select Intelligence committees are responsible for overseeing the performance of the Foreign Intelligence Surveillance Court. According to Michael J. O'Neill, chief counsel of the House committee, twice a year the panel privately reviews the petitions that have been approved by the court.

"We pay particular attention" to the court orders "that involve persons living in the United States," O'Neill said. "There are very few of them. But the committee wants to assure itself that all safeguards have been kept."

The bulk of the surveillance requests center on U.S. offices of foreign governments and the foreign nationals attached to them.

In these cases, the intelligence agency must show only that the office is controlled by a foreign power or that a foreign agent is involved in clandestine intelligence activity contrary to U.S. interests.

Presumably it was under one of these two guidelines that the United States invoked, at least in January 1980, in order to tap the phone calls being made to the Soviet Embassy here.

In order to show that Pelton, a former NSA analyst, had first contacted the Soviet Embassy in January 1980, the Justice Department had to play tape recordings of the conversations to the jury. That the United States was tapping the phone lines of the Soviet Embassy was one of several potentially embarrassing concessions that the government had to make to prosecute Pelton.

The request for such taps begins either with the FBI or the NSA and ultimately must be screened by the attorney general. The Justice Department contends that its perfect record in getting requests approved by the intelligence court is a result of the careful effort it invests in preparing requests and in weeding out those without sufficient cause.

However, the process is apparently not free of problems. According to a 1981 article in the Rutgers Law School Journal, the seven court judges were then spending little time in making sure that information gained from the electronic surveillance was confined to matters involving national security, terrorism or espionage.

The so-called minimization requirements are a critical part of the law, said Rutgers law professor Helene Schwartz, the author of the only academic study of the law.

Also, she concluded that the court should do much more to inform the public of its existence and the need for its existence.

"Weighing against the discreet activity required to protect the nation's most sensitive secrets is the necessity of reassuring the public about what is going on behind those heavy closed doors in the windowless room in the Justice Department," Schwartz wrote.

The "court has yet to strike that balance," she said.