Soviet Premier Alexei Kosygin, looking fit and hearty, reappeared today in public for the first time since he reportedly suffered a heart attack four months ago.

Kosygin, who is 76 today, delivered a 45-minute speech to a political gathering at the Bolshoi Theater. He clearly relished the frequent applause and offered his remarks with ease and a crisp delivery, tough he lost his place several times toward the end.

The premier struck the same note of harsh criticism of the United States as other Politburo members have in recent days and attacked his own country's economic performance, which has laged behind planned goals in recent years.

While great economic gains have been made recently, he said, "unsolved questions are quite numerous. Much is yet to be done to satisfy the population's demand for a number of foodstuffs and manufactured goods, to improve housing, medical and everyday services for the people."

As head of government, Kosygin is responsible for the economy, and such criticisms, have become routine as the ruling Communist Party seeks to improve performance without making fundamental changes that could diminish party control.

Kosygin, whose long absence has never been explained officially, was last seen publicly Oct. 17. His duties have been performed by Nikolai Tikonov, 74, who was made a voting Politubro member in November. b