April 21, 1984

The Reagan Administration has told six Eastern European Governments that they cannot hope for improved relations with the United States if they continue to support Palestinian and other ''international terrorists,'' State Department officials said today. The Administration said that if the six nations wanted better relations, they would also have to stop carrying out espionage and seeking to evade American export laws. The officials said the warnings were given separately during meetings last month between R. Mark Palmer, Deputy Assistant Secretary of State for European Affairs, and the ambassadors of Bulgaria, Czechoslovakia, East Germany, Hungary, Poland and Rumania. The Administration originally planned to keep the warnings secret, but East European diplomats here and abroad discussed them with reporters. This led the officials to provide details.

Twenty-five transit police officers moved aboard a subway train that pulled into the Delancey Street station early yesterday with its horn blasting for help and arrested 22 youths on charges of robbing and terrorizing riders. The transit police, who had responded to a radio call for help from an officer aboard the train, described their intervention as ''posse-like'' and ''so instantaneous'' that none of the dozens of riders were injured and none of the suspects escaped. The officers lined up everyone who had been on the train against the station wall and sorted out, from nearly 100 youths, those accused of having taken part in the robberies. 'He's the Kid' ''That's the one, he took my money,'' a rider shouted at one youth.

Silicon Valley has become so crowded that one of its leading lights, the Intel Corporation, is suing the city of Santa Clara to slow the growth. Not far ahead, people worry, with just a trace of exaggeration, are traffic and pollution as bad as in Los Angeles. The slump in video games has cost hundreds of jobs, and the belated but immensely successful entry of the International Business Machines Corporation into personal computers has shattered some companies here. Envied Abroad Nevertheless, for all its well-publicized growing pains, Silicon Valley is booming. The valley, a narrow, roughly 25-mile strip from Palo Alto to San Jose, remains the envy of the industrial world. Many foreign leaders have made it an obligatory stop - President Francois Mitterrand of France visited recently - and its freewheeling spirit is held out as the missing ingredient in older, declining industries.

The State Health Commissioner reversed himself yesterday and said he would allow the Memorial Sloan-Kettering Cancer Center to acquire a nuclear magnetic resonance scanner. The scanner, one of the world's newest and most expensive pieces of medical technology, provides intricate images of the body's internal organs and analyzes its chemistry without harmful radiation. The Commissioner, Dr. David Axelrod, recommended two months ago that Memorial Sloan-Kettering be denied a scanner, known as an N.M.R., because it could use a similar machine at the nearby New York Hospital-Cornell Medical Center. He said duplicating the one at New York Hospital with another at Memorial would further inflate health costs.

Two Soviet-built planes of ''unknown nationality'' fired on a United States Army helicopter today while it was on an observation mission along the West German-Czechoslovak border, the Pentagon announced. The Cobra helicopter was not hit by the rocket and cannon fire, the Pentagon said, and returned safely to its home station in West Germany. The brief announcement, which gave only sketchy information, said the Army's European headquarters in Stuttgart, West Germany, was investigating the incident, which occurred at 2:40 P.M. Mission Called Routine Pentagon officials said it was not known whether the helicopter entered Czechoslovak airspace or whether the MIG's entered West German airspace. (In Munich, an official of West Germany's paramilitary border police said there were ''indications that the helicopter was several kilometers into Czechoslovak territory'' when it was fired on. A United States Army spokesman in Stuttgart declined to comment on the statement.)