The first test of a pool arrangement for the news media to cover surprise military actions became known here yesterday hours after the Pentagon secretly flew a group of reporters to Honduras to observe U.S.-Honduran military exercises.

A Pentagon spokesman, Col. Robert O'Brien, publicly acknowledged the beginning yesterday of a test of the pool arrangement, set up after news organizations complained that they had been excluded from the Grenada invasion in October 1983. He said the spread of word about it among news organizations "gives a great sense of unease to the military-operations people."

O'Brien said that within hours after the 10-person pool was told Saturday evening to be ready to move out, one television network called, asking for details, followed by several other organizations he declined to name.

The Defense Department felt compelled to break its rules of secrecy to head off "speculative stories," he said. It informed the organizations that the pool was not going on a combat mission.

Defense Secretary Caspar W. Weinberger agreed in principle last year to organize a pool of reporters quickly to accompany U.S. invasion forces anywhere. In exchange, the Pentagon obtained assurances from major news organizations and press associations to keep any pool a secret until fighting began.

O'Brien said the word's spread would force the Pentagon to reconsider the plan.

He said defense officials had decided to test the agreement by establishing a pool to cover a large, previously announced military exercise in Honduras, Universal Trek '85, scheduled to get into high gear Tuesday.

O'Brien questioned whether the system is workable, given the competitive nature of U.S. news organizations and their intense scrutiny of U.S. military activities.

"We're trying to run a realistic exercise of the pool idea to make sure we can serve the media's needs and to make sure that the security of the mission is maintained," he said. "I think we have an answer to that. The net result is that security was breached."

O'Brien said the test began at 6 p.m. Saturday when eight news organizations were instructed to send reporters to Andrews Air Force Base by 4 a.m. yesterday. Officials said only that the journalists should be prepared for a week's stay in hot and rainy weather, O'Brien said.

The Pentagon said the organizations in the pool were United Press International, The Associated Press, Cable News Network, Mutual Radio, Newsweek Magazine, The New York Times, Copley Newspapers and Dow Jones.

Jonathan Wolman, assistant Washington bureau chief for The Associated Press, confirmed the AP's participation after the Pentagon did so, but said, "I have nothing to say about it. If anybody asks, we don't know anything, we can't say anything."

A spokesman for The New York Times also declined to comment. When told the Pentagon said his organization had a reporter on the expedition, the spokesman said, "That's news to me."

One Washington editor who asked to remain anonymous said, "I hate to raise this, but how are they so sure it leaked from a news organization and not from a government official?"

O'Brien said eight reporters plus two Cable News cameramen arrived at the base on schedule, were given inoculations, ate breakfast and were flown to Honduras for the maneuver, a 6,600-troop mission that is the largest ever in Honduras.

By 1:20 a.m. yesterday, O'Brien said, he had received a telephone call from a television network not included in the pool. O'Brien declined to name the organization. Similar calls from two other networks and three newspapers, including The Washington Post, were made to Pentagon officials, he said.

The Post's inquiry followed reports around noon yesterday from Edward Cody of the Post Foreign Service in Managua that rumors were circulating in Nicaragua about formation of a pool.

O'Brien said he felt constrained to acknowledge the pool's destination because "if I stonewalled, we were going to get some crazy stories about an invasion somewhere."

"Those stories are awfully hard to catch up with, and some news people aren't going to get the denial before it's too late," he said.