The first 40 of the planned 100 new MX intercontinental missiles will be based in existing Minuteman ICBM silos beginning in late 1986, the Pentagon announced yesterday.

It will take another 18 months for the administration to decide where it will base the additional 60 giant ICBMs, a Defense Department spokesman said.

The Air Force, at the direction of Defense Secretary Caspar W. Weinberger, is exploring three additional modes for basing the MX: putting them aboard a "continuous patrol aircraft"; burying them 3,000 feet deep in underground silos termed "citadels"; or putting them in additional fixed or "deceptively based" silos and protecting them with a new anti-ballistic missile defense system.

Cost of developing and buying the first 100 operational MX ICBMs, plus the additional 126 spare and test MX missiles that go with them, will be about $13.8 billion. The cost of basing them in Minuteman silos will add another $5.6 billion, a defense spokesman said, putting the total cost of the initial MX program between $18 billion and $19 billion.

On Oct. 2, President Reagan announced he was cutting President Carter's proposal to build 200 MX missiles back to 100. The president also said he was canceling the former administration's plan to rotate the giant ICBMs among 4,600 shelters in Nevada and Utah.

At the time of the president's announcement, Weinberger said the plan was to base the first 36 MXs in silos that now hold the aging Titan II missiles. The Titans are to be retired beginning late next year.

A Pentagon spokesman said yesterday the use of Minuteman rather than Titan silos for the MX will save $1.5 billion, because of the additional costs of adapting the older silos.

The Minuteman fields--which now hold over 100 missiles apiece--are also better spaced, the spokesman said, if the administration goes to a ballistic missile defense system because they will permit all the proposed MXs to be based in one area.

The Carter MX plan was expected to cost over $50 billion and created a political uproar in the states where the missiles were to be located.