The Reagan administration would forgo deployment of the MX missile only if the Soviets dismantled all 650 of the biggest missiles they already have in the field, Arms Control and Disarmament Agency director Kenneth L. Adelman told Congress in a letter released yesterday.

Senate Foreign Relations Committee Chairman Charles H. Percy (R-Ill.), divulging Adelman's written response to a question asked a week ago, said it was "the first time the administration has publicly indicated that it would consider a 'fair trade' for the MX."

But Democratic and some Republican committee members were unimpressed, saying Moscow would never agree to such a trade involving dismantling of all its SS18 and SS19 missiles and that it is typical of the un-negotiable arms control proposals the Reagan administration tends to make.

"The letter from Adelman is a perfect reflection of the attitude of this administration," said Sen. Joseph R. Biden Jr. (D-Del.). "We will give up something if they give up everything."

Sen. Paul E. Tsongas (D-Mass.) said the Adelman response, in combination with something Secretary of State George P. Shultz told the committee last week, illustrates why people do not believe the administration is sincere on arms control.

Tsongas pointed out that Shultz, in a long statement on U.S. policy toward Moscow, had said all strategic weapons "are on the table" in negotiations. But the Adelman letter, Tsongas said, makes clear that "MX is not, has never been and will not be a bargaining chip" in those negotiations.

Sen. William S. Cohen (R-Maine) called the Adelman proposal "commendable but impractical. The Soviets will reject it out of hand. The Soviets have historically rejected radical restructuring of their forces and will reject this one." Cohen said there are similar problems with the broader U.S. proposal at the strategic arms reduction talks (START) in Geneva, which also seek major cuts in Soviet weapons.

Sen. Sam Nunn (D-Ga.) also called the MX trade-off terms impractical and recounted how an eastern European diplomat once explained to him why the Russians were cool toward such proposals as President Reagan's zero-zero plan to remove all Soviet missiles from Europe in return for a U.S. pledge not to put any there.

"You have 15 aircraft carriers," the east European said, "and we have plans to build an aircraft carrier. We will not build our carrier if you promise to sink the 15 you already have."

The Reagan administration has periodically shifted its stand on what if anything it might trade for the MX. Several officials, including Reagan in a letter in January to Rep. Jack Kemp (R-N.Y.), have said the MX is not to be bargained away but is needed to modernize the U.S. land-based missile force. What is negotiable, officials have said, is how many MXs this country might ultimately deploy.

However, on occasions when the administration is seeking support for new weapons while at the same time trying to portray itself as sincere about arms control, it says such things as "everything is on the table."

Last week, at a closed-door committee session, Adelman was asked whether there were any circumstances in which the United States might give up the MX.

His response:

"The president has made clear that the scale of the MX deployment will be influenced by Soviet strategic programs and arms reduction agreements. The MX is the U.S. response to a massive buildup of Soviet ICBMs over the last 10 years, and unless the Soviets are prepared to reverse this buildup and forgo their heavy and medium ICBMs, the U.S. will go forward with MX."

At yesterday's Foreign Relations hearing on arms control, the White House was also criticized for its reported lack of enthusiasm for a so-called "guaranteed build-down" plan for reducing nuclear arms offered by Cohen and Nunn. This involves withdrawing more than one old warhead for every new one deployed. The president has endorsed the idea but aides have said more time is needed to work out details.

Cohen testified that "many of the people the president has charged with developing a build-down concept do not appear to share his positive commitment to the concept." Cohen complained that these officials "either lack understanding or desire," and warned that he will withdraw his support for the MX missile unless the White House moves soon to come up with a clear formulation.

If Reagan backs away from build-down, it could cost him the continued support he needs from key congressional moderates for the MX.

The build-down proposal, however, has many opponents, in the Pentagon and State Department as well as the White House. Percy, who supports build-down, told Cohen and Nunn that in Illinois it is also opposed by many who see it either as unilateral disarmament or a figleaf to cover an arms buildup that lets MX go ahead.