FBI agents in Tampa, Fla., last night arrested a former U.S. Army sergeant on charges that he worked for a West Germany-based spy ring that passed NATO defense plans and other classified documents to the Hungarian and Czechoslovakian intelligence services.

Roderick James Ramsay, 28, who served in the U.S. Army's 8th Infantry Division from 1983 to 1985, was charged with espionage in a criminal complaint released by the U.S. attorney in Tampa. Ramsay, who resides in Tampa, was allegedly recruited by another former Army sergeant, Clyde Lee Conrad, the alleged leader of the spy ring, who was convicted of treason and sentenced to life in prison by a West German court earlier this week.

FBI Director William S. Sessions said the classified information divulged by the spy ring was "extremely sensitive" and that the investigation was one of the "most complicated foreign counterintelligence investigations ever conducted by the FBI."

An affidavit in support of the criminal complaint that is due to be released in federal court today will charge that in exchange for $20,000, Ramsay gave Conrad classified materials, including "General Defense Plans" for the defense of Central Europe, documents dealing with use of tactical nuclear weapons by the United States and NATO allies, documents coordinating various NATO forces, technical manuals and details of military communications technology.

The affidavit will charge that these papers were then passed to the Hungarian and Czechoslovakian intelligence services, officials said.

The affidavit will also charge that the Conrad spy ring used a cow bell as its recognition signal. After recruiting Ramsay as a spy, Conrad gave him a cow bell, and told him that if anyone displayed a similar cow bell, Ramsay would know that person was involved in Conrad's espionage activities, officials said the affidavit will charge.

FBI officials said last night that the investigation was continuing and other Americans may be arrested. "This arrest is the tip of an iceberg," said bureau spokesman Larry Curtin.

Ramsay, who was arrested without incident, is scheduled for a court hearing today.

Federal officials said that as an assistant document custodian of the G-3 Plans section of the Army's 8th Infantry Division in Bad Kreuznach, West Germany, Ramsay held a top secret clearance and was responsible for safeguarding and accounting for all classified documents. The division is a mechanized unit that includes some of the Army's newest weaponry, including the M-1 tank and the Bradley Fighting Vehicle. As part of the Army's 5th Corps, it has responsibility for defending central West Germany in event of an attack from the Soviet bloc.

In one alleged incident about December 1985, officials said, Ramsay videotaped hundreds of documents over the course of a week, then made the tapes available to Conrad for eventual sale to Hungary and Czechoslovakia.

Conrad was arrested by West German authorities in August 1988. Seven other people were also arrested in Western Europe, including two Swedish doctors of Hungarian origin and one of the doctors' wives, who allegedly served as couriers. At the time, officials said it was difficult to assess how much damage the spy ring had caused because it was unclear how much classified material was accessible to Conrad.

But in the course of the joint FBI-U.S. Army intelligence investigation, as well as Conrad's recent trial in West Germany, additional details have been uncovered that suggest the espionage may have been of a more serious magnitude. Conrad, a native of Sebring, Ohio, had worked for the two Warsaw Pact intelligence services for up to 10 years, in 1975-85, and received $1.2 million.

In reading his judgment at the conclusion of Conrad's trial this week, Chief Judge Ferdinand Schuth said that Conrad had "endangered the entire defense capability of the West." If war had broken out, he said, the information passed to the East "could have led to a breakdown in the defenses of the Western Alliance" and to "capitulation and the use of nuclear weapons on German territory."