For more than 20 years, the CIA and Russian intelligence agencies have had a back-channel relationship, much like the one U.S. officials are now using to reduce the number of Russian agents in this country.

The secret meetings were held when one side feared the other had wandered "out of bounds" in its clandestine activities, according to a former intelligence official.

For example, CIA Director William E. Colby in 1976 directed CIA officials to meet with the KGB to determine whether the Soviet spy agency had any role in the 1975 assassination of Richard Welch, the CIA station chief in Athens. At the session in Vienna, the CIA officers present "made it clear we [the agency] won't stand for it," one source said of the session. The senior KGB officer was angered, but the Soviet intelligence service "remembered the representation," the source said. The killing was done by a Cypriot terrorist group.

After the 1984 kidnapping of William Buckley, the CIA station chief in Beirut, then-CIA Director William J. Casey ordered a session with the KGB to see whether the Soviets could help uncover what had happened. "There was no evidence they were involved in the case," a source said of the Vienna meeting.

Thereafter, the channel was used when one side or the other had a problem. The Soviets "persuaded themselves we were drugging and kidnapping their officers," a retired senior CIA official said. One had been killed in Peru "and they thought we had been too aggressive," he added.

In December 1987, when Soviet President Mikhail Gorbachev visited President Ronald Reagan in Washington, then-acting CIA Director Robert M. Gates had dinner with his opposite number in the KGB, Vladimir A. Kryuchkov.

By 1989, with the Iron Curtain collapsing, a more formal channel opened, managed by Milton A. Bearden, then chief of the Soviet Division within CIA's clandestine Directorate of Operations. It became so formal that a secure phone line was established between CIA headquarters in Virginia and KGB headquarters in Moscow.

During the Persian Gulf War buildup in 1990, discussions between the intelligence services expanded to include cooperation against Iraq and talk of reducing operations against each other, one source said.

On the eve of German unification in October 1990, Bearden met in East Berlin with his Soviet counterpart and listened to concerns about KGB officers defecting to the West. Bearden's critics within the agency say he began reducing the number of defectors the CIA would support. One turned away for lack of agency interest was former KGB archivist Vasili Mitrokhin Vasili, whose book on the KGB, "The Sword and the Shield," was just published.

In July 1991, Bearden brought a CIA delegation to Moscow to discuss arms proliferation, chemical and biological weapons, and drugs. Eventually there was public cooperation on drugs, terrorism and nonproliferation because of the nuclear weapons in the old Soviet Union. "They learned that narcotics were not just a U.S. problem," one former official said.

Gates said he met secretly again with Kryuchkov in 1991, when he accompanied then-Secretary of State James A. Baker III to Moscow. Gates said he tried, unsuccessfully, to get Moscow to release the wife and family of Oleg Gordievsky, the London KGB chief who had been recruited as a spy by the British and was spirited out of Moscow in 1985 by MI6.

CAPTION: Then-CIA Director William E. Colby in 1975. He ordered CIA-KGB meetings.