The Reagan administration, in an effort to shut off leaks and ensure that the government speaks with one voice on national security matters, had adopted a press relations policy that appears to be considerably more restrictive than those of previous administrations.

The Central Intelligence Agency, which in recent years occasionally provided unclassified background briefings to reporters on request on a variety of subjects from oil to Afghanistan, has now ended that policy on orders from new Director William J. Casey.

The top-level National Security Council staff in the White House, including dozens of specialists who also frequently provided background information on defense and foreign policy subjects in previous administrations, is now off-limits to reporters on orders of the president's national security adviser, Richard V. Allen.

At the departments of State and Defense, many career officials still welcome reporters to their offices on an informal basis. And there are still occasional background briefings to provide a fuller account of administration views on issues in which the officials doing the briefing cannot be identified by name.

But in those two agencies, other important information sources have gone dry. These are the daily public briefings by department spokesmen where reporters have one of the few chances to get the administration to explain policies on the record.

By the account of many experienced reporters, these regular briefings at both State and Defense are at their least productive point in many years in the information they yield or the opportunities to extract more than what amounts to a daily government press release.

At the State Department, spokesman Dean Fischer, a former newsman, arrives for the briefing with a sheaf of papers each day containing official department "guidance" that is prepared by senior specialists on a variety of subjects. When a question is asked for which he has guidance, he turns to that sheet in his folder and reads it. Nothing more is divulged.

One news agency reporter noted that during a regular 45-minute noon briefing at State recently, the spokesman responded more than 30 times with "don't know, can't say, no comment" or "I've got nothing for you on that."

Asked by The Post if the narrow scope of his guidance, which keeps him from being informative, bothers him, Fischer said: "The most difficult part of my job is to stand up there and say virtually nothing, particularly about the Middle East. But that's not a complaint about the guidance because I fully understand the reasons for it."

Those reasons, he says, are to avoid anything that could jeopardize U.S. diplomatic efforts.

At the Pentagon, spokesman Henry E. Catto Jr. was neither a newsman nor a defense specialist before taking over public affairs for an agency dealing with some of the most specialized issues in government. The tone for Pentagon information policy appeared to be set by Catto during his first formal briefing May 19 when he told reporters there would no longer be any detailed accounting of U.S. and Soviet naval power in the Mediterranean, information that was routinely given out by previous administrations, even in a crisis.

A month earlier, Deputy Secretary of Defense Frank A. Carlucci put out a chilling memorandum to the Pentagon bureaucracy warning that unauthorized disclosure of classified information, whether inadvertent or intentional, would not be tolerated.

While that may seem normal, Carlucci also referred specifically to espionage laws and added that "even unclassified matters should be treated with circumspection when they relate to sensitive internal deliberations."

In circulating Carlucci's memo to his public information staff, Catto added a note that said aside from "posing a threat to national security, unauthorized disclosures tend to make our work more difficult by stimulating inquiries about the subject matters revealed. I am particularly concerned that there be no wounds of this type inflicted by members of this office."

The combination of these factors -- the shut-off of access to CIA and NSC experts, the intimidating tone used toward the bureaucracy and the low information content of public briefings at State and Defense -- may seem within the Reagan administration to have advantages in terms of controlling information flow.

It also may be convenient for an administration which has not formed policies on such key issues as relations with the Soviet Union and arms control or, for that matter, has not had a major public speech by the president describing his foreign policy.

For reporters and newspaper readers, however, the effect is a thinner explanation of the reasons certain things are happening in foreign policy, defense and national security affairs and what alternatives are being considered or have been overlooked.

There is also a potentially serious loss of accountability to the public when spokesmen for two agencies that deal with matters of national life and death are unable to explain the actions of their leaders.

The new administration clearly has been trying to avoid the kind of open confusion that occurred during the Carter regime because of publicly aired differences between Secretary of State Cyrus R. Vance and national security adviser Zbigniew Brzezinski. This is understandable and explains why Secretary of State Alexander M. Haig Jr. does the talking on U.S. foreign policy publicly as opposed to Richard V. Allen.

White House chief of staff James A. Baker III, in a recent interview with The Washington Post, said one of the initial problems the new administration had was in making sure national security information was disseminated "with one voice and in a coordinated way" and that this problem "was now under control," except for the leaks that have troubled every administration.

Those leaks, he says, are a problem for a number of the president's foreign policy initiatives.

Among other steps that have been taken, Baker said, the administration has cut the number of people attending National Security Council meetings and "suggested" to NSC staff members that they not talk.

But by putting the staffs of the NSC and the specialists in CIA also off-limits to reporters, the new administration also has removed two places where reporters could find people exposed to the broadest, government-wide view on many issues.

These staffs generally are not subject to the kind of bureaucratic influences that go with the Pentagon's view of the world and the huge military budget, or the State Department's primary concern with diplomatic factors.

A White House press officer, who asked not to be quoted by name, claims that while there is an administration-wide concern about shutting off leaks, there is no administration policy shutting off reporters. In essence, he argues, whatever actions have been taken have been individual ones.

CIA director Casey concluded that the agency spent too much time on background briefings, that it wasn't part of the agency's mission to brief the press, and ordered a halt late in March.

Allen claims reporters technically aren't shut off, yet requests for interviews with staff members are not being granted and if they were granted, policy could not be discussed and the interview would be off-the-record, which means it could not be used.