The Pentagon's first experiment with a system designed to permit media coverage of secret military operations resulted in leaks Sunday and an article yesterday in The Washington Post after Mutual Radio network spread word of the exercise.

Mutual told other radio networks that the Pentagon had activated a prearranged "pool" of newspaper, radio and television reporters shortly after being asked to send a correspondent to Andrews Air Force Base by 4 a.m. Sunday to cover an unspecified operation.

When a Post correspondent in Nicaragua learned that a radio colleague had been told by his home office that the pool was activated, the correspondent told his editors. The Pentagon subsequently confirmed that a pool test had been initiated Saturday night.

Mutual told the others despite a Defense Department directive to keep the plans secret.

Bart Tessler, Mutual's news director, said he acted under guidelines radio executives had given the Pentagon last fall. He said that Defense officials never responded with approval or disapproval of the guidelines.

Defense Secretary Caspar W. Weinberger approved the pool plan last year to ensure news media coverage of surprise U.S. military operations such as that in Grenada, where the press was excluded for 2 1/2 days in October 1983. The plan's first test -- to cover a scheduled military exercise in Honduras -- drew criticism from Pentagon officials and media spokesmen alike as both sides spotlighted flaws.

Pentagon spokesman Michael I. Burch said the Defense Department would review the system and possibly stage another test to iron out difficulties.

"We hoped it would prove the case that we could confide in the press to cover the exercise," Burch said. "There was a breakdown in operations. We've got to look at it. The press has got to look at it, and we have got to see how we can do it better in the future."

Media executives said that many pool details had not yet been worked out with the Pentagon; they saw the test as somewhat premature and worried openly that the rapid spreading of the word over the weekend would strengthen the hand of those who want to exclude reporters from military operations.

After journalists were barred in Grenada, most major news organizations issued strong protests, prompting the Pentagon to set up a panel to study modern war coverage. The Weinberger-approved plan was designed to include a pool of eporters who would keep the mission secret until it began.

"When there are flaws, as there are in the present pool system, all parties should work to correct those problems. That is what tests are for," said Charles J. Lewis, Washington bureau chief for The Associated Press, which provided a photographer to the pool. "It's premature to write the obit of the Pentagon pool system."

"I'm not sure this is going to go down as a failure," said Robert D. McFarland, vice president and Washington bureau chief of NBC News. "I think this will go down as a test that had some problems."

At 6 p.m. Saturday, Pentagon officials began calling executives of eight news organizations chosen to cover the Honduras exercise. They were told to be prepared for "70- to 90-degree" weather and rain. They were not informed of the destination or that the pool was a test.

Pentagon officials said that they read to the executives a prepared advisory, including the warning that "secrecy, until the operation begins, is paramount. Any leaks could jeopardize the operation. You may not discuss with anyone that the pool has been activated."

The pool representatives arrived in Honduras around noon Sunday, according to the Pentagon.

Word of the test actually emerged several days in advance. McFarland said that NBC learned from "Pentagon sources" Wednesday or Thursday that the pool exercise was imminent.

Pentagon spokesman Col. Robert O'Brien said that, as far as the Defense Department was concerned, the secrecy agreement was broken Sunday at 1:20 a.m. when a network bureau chief called him.

Jack Smith, CBS News Washington bureau chief, acknowledged yesterday that he made a call at about that time because he had heard of the mission from a source and, as the chief of the pool operation for the networks, he had been waiting to hear from the Pentagon.

"I told the Pentagon officer that I was aware of the pool, CBS News had learned that a Pentagon pool had been activated and I wanted to know why was CBS not notified," Smith said. "Why was not the pool chief notified?"

He said network officials had been led to believe that each network would be allowed a correspondent on a larger pool such as this one. He added that, after talking to Pentagon officials and learning it was a test pool, CBS did not broadcast news of the exercise until it had appeared Monday in The Post, not a pool member.

At Mutual, Ron Nessen, vice president for news, got the call from the Pentagon, according to Pentagon officials. Nessen was unavailable yesterday, but Tessler said Nessen's notes did not include a warning not to tell other radio bureau chiefs.

As a result, Tessler said, he telephoned eight other bureau chiefs Sunday morning to order the necessary phone lines for a possible radio feed from Mutual's pool reporter. He said that he told them the operation was to be kept secret.

Tessler said there had been an agreement within the radio industry in Washington that whenever one company joined a pool, other companies would be notified so they could arrange for transmission facilities. He said this agreement was sent to the Pentagon for comment last year but got no reply.

Burch said that The Post was "not at fault" for publishing the story yesterday, because the details were publicly acknowledged by a Pentagon spokesman after Smith of CBS called.

Several members of the pool said they were unhappy about the Pentagon's "lifting the lid" of secrecy early Sunday without informing them that they were freed of their promises not to publish anything.

Lewis of the AP said the first time he knew that the pool was an exercise and that the lid had been lifted was when The Post called and asked for comment.