The aged, accident-prone Titan II missiles are being kept in service, military and civilian technicians who work at the sites believe, because "they are the missiles the Soviets fear most."

But there is evidence the Titans are still around because in 1972 the Soviets feared them less than other types of missiles that might have replaced them.

Under the SALT I agreement published that year, the United States had the right to trade the 54 Titan IIs for three new missile-launching submarines. But before Soviet Premier Leonid Brezhnev signed that agreement, sources said, he wanted written assurances from President Richard M. Nixon that the United States would not exercise that option.

In a secret, written "assurance," sources involved in the negotiations said, Nixon told the Soviets he had "no intention" of replacing the Titan IIs with new submarine-launched missiles. In effect, Nixon froze the already 9-year-old missiles into permanent operational service.

"The Soviet concern was not with the Titans," a member of the U.S. negotiating team said recently, "but that we would get rid of them for [sub-launched missiles]."

William Hyland, who worked with Henry A. Kissinger in writing the final SALT language at a Moscow summit in 1972, said yesterday that he recalled "we gave them something in writing" on the Titan, but he did not remember "if they asked or we volunteered it."

Hyland did recall Brezhnev was worried that the Trident submarine, which was then in early planning stages, would "flood the oceans with hundreds and hundreds of [multiwarhead] submarine missiles."

The original notion was to trade off the Titans for the biggest missiles in the Soviet arsenal, the SS9. Some of those giants carried a 25-megaton warhead.

But the Soviets would not buy any deal with their SS9, so a new arrangement was devised.

The U.S. Titans and two early Soviet ICBMs, the SS7 and SS8, were termed "older types." Agreement was reached that "older types" could not be replaced by modern land-based ICBMs. Each side could dismantle these "older types" and replace them with an equal number of new sub-launched missiles.

The Soviets ended up doing just that. Since SALT 1 was signed, they have destroyed all 209 of their SS7s and SS8s and added an equal number of sub-launched missiles.

The 1972 agreement permitted the United States to have up to 44 strategic missile submarines -- three more than the 41 Polaris and Poseidon subs then operational.

But the United States has never gone over 41. In fact there are plans now to dismantle two Polaris submarines so that the SALT I and SALT II limits are not breached when the first Trident sub becomes operational later this year.

Hyland yesterday said he believed the Nixon "assurance" that the Titans would not be replaced ran only for the five-year term of the 1972 protocol. He suggested it was possible under the SALT agreements to take down the Titans now and replace them by keeping in operation the two Polaris subs scheduled for deactivation.

Former Defense Secretary Melvin R. Laird said last week he "had heard" of the Nixon letter to Brezhnev about the Titan but had never seen it.

In Laird's words the Titan thereafter became "one of those systems that just stood around" without having a meaningful mission.

The SALT focus on overall power made it "politically foolish," according to Hyland, for anyone to suggest dismantling the Titans without getting some concession from the Soviets.

One reason for keeping the Titans, according to Hyland, was "so we could count something."

Their warheads, at nine megatons apiece, were far larger than those of the Minuteman II (one megaton) and meant that 54 Titans represented about one-third of the total explosive power of the U.S. landbased ICBMs.

In the years after the SALT I agreement and the Nixon "assurance," the Pentagon studied several ways to upgrade the Titans, since the 1972 SALT agreement with the Soviets barred the United States from replacing them with a more modern intercontinental ballistic missile.

One study, according to a former government official, was directed at providing the Titan with more than one bomb -- making it a so-called MIRVed missile. "They decided it wouldn't pay to do that for only 54 missiles," the source said.