The CIA has decided to go underground again.

It is no longer necessary, CIA Director William J. Casey announced in a newsletter circulated in the agency last week, to spend much time justifying the agency's activities or defending the quality of its work. In his view, "the difficulties of the past decade are behind us."

As a result, contacts with the press and public, which have already been cut back, will be reduced still further. The CIA's office of public affairs will be closed and its work assigned to a new section that will also take over legislative liaison.

Both assignments have had high-level status since disclosures of CIA domestic spying and other misdeeds prompted a series of executive branch and congressional investigations in the mid-'70s. That was a time, Casey noted, when the agency "was still encountering considerable criticism in the media and the Congress and when it was important to expend considerable effort" explaining and defending the agency's work.

Apparently confident that a "trust us" atmosphere has returned, the CIA director said he feels "the time has come for CIA to return to its more traditional low public profile and a leaner -- but no less effective -- presence on Capitol Hill."

The head of the office of public affairs since 1977, former Navy captain Herbert E. Hetu, reportedly disagreed with the decision to downgrade the work and will be leaving the agency as a result of the shuffle.

The reorganization comes on the heels of Casey's March orders to halt the occasional background briefings took up a lot of time and were not a proper undertaking for an intelligence agency.

The CIA's office of legislative counsel, headed by Fred Hitz, also will be downgraded. Its legislative liaison duties, along with the public affairs work of Hetu's staff, will be taken over by branches of a new office of policy and planning under CIA veteran Robert M. Gates. The work of drafting legislative proposals and analyses will be turned over to the CIA general counsel's office.

Gates will report directly to Casey and CIA Deputy Director Bobby Inman. He will also retain his present duties as their executive staff director for the intelligence community.

The change will take effect tomorrow and could result in a reduction in the issuance of public reports and studies under the CIA imprimatur. Inman, who formerly headed the supersecret National Security Agency, is known to feel strongly that intelligence agencies should not have a public role.

"Our emphasis from now on should be tomaintain and enhance CIA's reputation not by representational activities but by the excellence of our work and the high quality of our contribution," Casey said in last week's circular, "From the Director."

In that vein, Casey said he was also taking steps to improve the CIA's national intelligence estimates, which are supposed to be comprehensive studies of an issue or subject reflecting the views of the entire intelligence community.

Casey said the process of preparing them had become "slow, cumbersome and inconsistent with providing the policymaker with a timely, crisp forecast that incorporates clearly defined alternative views."

For years there has been some bitterness in the intelligence community because the antional intelligence estimates have been predominantly CIA products, with dissents from other agencies usually relegated to footnotes.

Casey said he had decided to organize the cadre of national intelligence officers, who are in charge of drawing up the estimates, into a formal National Intelligence Council. The council will report directly to Casey and Inman and the council's chariman, who has yet to be named, will serve as a chief of staff over the other officers.