WASHINGTON (AP) _ The Agriculture Department's chief negotiator with Soviet grain officials collapsed at a congressional breakfast today, a department spokesman said.

Undersecretary Daniel G. Amstutz was taken to a hospital emergency room, said John McClung, USDA director of information. The talks with Soviet Union officials began on Monday and were to have concluded today. McClung said Amstutz's aides took over the meeting.

McClung said he had no immediate word on the cause of Amstutz's seizure. However, Amstutz reportedly showed rapid improvement after being attended to by emergency personnel, he said.

Mike Masterson, an aide, said Amstutz's illness ''doesn't seem to be a heart problem'' and that he was expected to be released soon, perhaps ''in a couple of days.''

Masterson said Amstutz became faint at a USDA breakfast with members of the House Agriculture Committee and was admitted to George Washington University Hospital after emergency treatment.

The closed-door meeting with the Soviets was taken over by two Amstutz deputies, Richard W. Goldberg and Thomas O. Kay, Masterson said.

The meeting with the Soviets was scheduled under terms of a long-term agreement calling for regular annual purchases of U.S. grain.