Alexei Semenov said yesterday he doubts that his parents, Andrei Sakharov and Yelena Bonner, have ended their hunger strike in return for a promise by Soviet authorities to let his wife, liza Alexeyeva, come to the United States.

Semenov, Sakharov's stepson, reacted with happiness and skepticism to the announcement that Alexeyeva would be permitted to join him.

"The KGB is not known always to keep its promises," Semenov said, adding that he doubted that "the hunger strike was stopped."

"I doubt very much that my mother and father would believe a message from the KGB," Semenov, a 25-year-old graduate student at Brandeis University, told reporters.

Sakharov, a physicist who won the 1975 Nobel Peace Prize for his work on behalf of human rights, and Bonner stopped eating Nov. 22, hoping to pressure authorities into allowing Alexeyeva, who married their son by proxy last June, to leave the Soviet Union.

President Reagan, meanwhile, was described as "pleased" and "gratified" about the Kremlin's decision to let Alexeyeva, 26, emigrate.

The president "is gratified by the decision to grant an exit visa to liza Alexeyeva and he is pleased with reports that the Sakharov's hunger strike is ended," said Larry Speakes, deputy White House press secretary.

"It is our hope they will be allowed to lead a normal life," Speakes said.

State Department spokesman Alan Romberg told a news conference: "We can confirm reports that the KGB told Alexeyeva that she would get an exit visa and that Sakharov has been informed of this."

Sen. Daniel Patrick Moynihan (D-N.Y.) called the move the result of a "worldwide demand for decency."

And Sen. Paul E. Tsongas (D-Mass.) said he was "very gratified" by the Moscow announcement, but added: "It is not yet time, however, to celebrate."

He noted that the granting of the visa was conditioned on Alexeyeva not meeting with foreigners and said "this could delay the outcome."