LAS VEGAS, JAN. 30 -- Soviet and U.S. scientists said today that the superpowers might reach agreement by the end of the year on how to verify the size of each other's nuclear explosions.

Physicist Robert Barker, head of the U.S. negotiating team, and chief Soviet delegate Igor Palenykh told a news conference concluding a Soviet visit to the Nevada Test Site that the subject of verification would be the primary topic when arms-control talks resume in Geneva.

They said agreement on verification eventually could lead to a U.S.-Soviet nuclear test ban treaty.

Palenykh and Barker said they hoped joint Soviet-U.S. nuclear weapons tests would be conducted this May while scientists are at each other's test facilities experimenting with verification equipment.

The 20 visiting Soviet scientists left for Moscow shortly after the news conference, concluding an historic exchange between the superpowers dictated by a summit agreement between President Reagan and Soviet leader Mikhail Gorbachev.

U.S. scientists completed a visit two weeks ago to Semipalatinsk, the top-secret Soviet nuclear test faciity in southeast Asia.

"We have full-scale negotiations in nuclear testing," Palenykh said. "It must involve the whole range of banning nuclear explosions."

Neither nation revealed nuclear weapons secrets nor were nuclear weapons tests conducted during the exchange. Soviet scientists toured portions of the 1,350-square-mile Nevada site but did not see the underground tunnels of Rainier Mesa, where experiments have been conducted on the Strategic Defense Initiative.

The short-range goal of a verification agreement would be Senate ratification of the Threshhold Treaty and the Peaceful Nuclear Explosives Treaty negotiated in the 1970s, which impose a 150 kiloton ceiling on all tests. The long-range goal would be a nuclear test ban treaty, the scientists said.

"There is no doubt that the visits were a step toward our ultimate goal, but many other steps are required," Barker said.

Barker said arrangements will be made for 40-member teams of U.S and Soviet scientists to visit each others top-secret nuclear weapons sites for two months to experiment with different verification techniques during joint tests.

"We are most optimistic about having joint {nuclear} experiments in May and completing {verification} negotiations by the end of the year," Barker said. However, he said, if more than one joint test were required, the timetable would be revised.