The Senate met Tuesday in an unusual closed-door session called by conservatives who say the Soviets are lying about their nuclear forces, but supporters of the U.S.-Soviet arms treaty said the charges already have been refuted.

"We're going to examine the evidence that there are over 1,000 SS-20s, that the CIA has attempted to cook the books on the issue, and that the Soviets have deliberately falsified their data," said Sen. Steven Symms, R-Idaho, one of the chief critics of the treaty banning medium-range missiles.But Sen. William Cohen, R-Maine, vice chairman of the Intelligence Committee, said the charges are "the same old stuff" that has been considered and rejected by the panel as it endorsed the treaty last week.

The Soviets have declared that they have 650 SS-20s and all those missiles will have to be destroyed when the pact takes effect.

But Symms and other critics say the Soviets have up to 350 SS-20s more than they have declared, and the critics have said U.S. intelligence estimates of the higher numbers have been hidden.

"In the opinion of this senator, the CIA has cooked the numbers of SS-20s and I'm prepared to prove it," said Sen. Malcolm Wallop, R-Wyo., another critic of the pact.

Sen. Patrick Leahy, D-Vt., a supporter of the treaty, said, "These arguments have been heard and rejected. I don't expect to hear anything I don't know."

The Old Senate Chamber where the meeting was held has only rarely been used for business meetings since the Senate moved to its present quarters in 1859. It was used this time because the regular Senate chamber is laced with modern broadcasting and electric cable that could make eavesdropping possible.

It is the first secret Senate meeting since Oct. 7, 1986, and one of six in the past six years.

Senate Majority Leader Robert Byrd, D-W.Va., said Tuesday that the closed-door meeting was sought by conservatives, and he noted that Senate rules permit such a session at the request of any two senators.

Supporters of the U.S.-Soviet treaty say the Senate Foreign Relations Committee is likely to approve it by a margin similar to the 18-2 endorsement given Monday by the Senate Armed Services Committee.

The Foreign Relations panel is scheduled to vote Wednesday on whether to send the pact to the floor of the full Senate, where approval is expected next month.