The Reagan administration, in deciding recently to provide sophisticated Stinger antiaircraft missiles to guerrilla fighters in Afghanistan and Angola, made two key assumptions, according to administration sources: that news of the decision would not leak to the public and that none of the Stingers would fall into the hands of terrorist organizations bent on downing a commercial airliner.

The first assumption already has failed. As the first Stingers were being carried by military transport to U.S.-backed insurgents in distant bush wars last week, President Reagan's decision was widely reported by U.S. news organizations. Dozens of members of Congress and their staffs had been briefed on the decision, and dozens of officials in the State and Defense departments and the White House are involved in implementing the decision.

Now some experts inside and outside the administration are concerned that the second assumption -- the possible diversion of a Stinger to a terrorist group -- is also in jeopardy.

One Senate Republican aide pointed out that there were serious concerns in Congress about providing Stingers to the mujahadeen rebels in Afghanistan. "Some of these guys are a lot closer politically, religiously and philosophically to [Iran's Ayatollah Ruhollah] Khomeini than they are to us," the Senate aide said. "There is concern that one of these guys could show up in Rome aiming a Stinger at a jumbo jet."

Two years ago, the Reagan administration abruptly dropped its plans to provide hundreds of Stinger missiles to moderate Arab states in the Middle East because of congressional concerns over whether they might be diverted to terrorist organizations or used against Israel.

Officials of the American Israel Public Affairs Committee (AIPAC) called the Stinger the "ideal terrorist weapon" and opposed giving such sophisticated heat-seeking missiles to potentially unstable Arab regimes that lacked the security to prevent the missiles from falling into terrorist hands.

The Stinger is highly effective and highly portable. Its advanced heat-seeking technology allows its operator to fire at an oncoming aircraft from more than five miles away, leaving the maximum time for escape and for avoiding detection.

In May 1984, having failed in initial efforts to ship Stingers to Saudi Arabia and other Arab states, Reagan used emergency powers to provide 400 of the missiles to the Saudis. The deal was contingent upon firm assurance from the Saudis that strict security would be maintained and that U.S. military personnel would accompany the missiles to train the Saudis and to provide an additional layer of security.

AIPAC has not taken a position on the newest Stinger initiative, but the organization still holds the strong view that as a general matter, Stingers are "the terrorist's delight," as one official said yesterday.

An aide to Senate Majority Leader Robert J. Dole (R-Kan.) added, "The question of diversion has some relevance with respect to Afghanistan . . . but we were fairly comfortable about the end use in Angola" where Stingers are being provided to the guerrillas of Jonas Savimbi, who is facing what is expected to be a heavy spring offensive by Soviet- and Cuban-backed forces.

In the interagency deliberations that led to last month's Stinger decision, concerns about diversion to terrorists were overcome by the argument that anticommunist resistance forces in Afghanistan and Angola were in such dire military need of the antiaircraft missiles that there was little likelihood any would find their way into terrorists' hands, according to officials familiar with the discussions.

"You're talking about the crown jewel of their [rebels'] arsenal," said one administration official.

Still, some administration critics predicted that sending Stingers is likely to have repercussions in the Arab world, where some governments will wonder why the Afghan mujahadeen and Angolan rebels are more trustworthy and deserving than longtime U.S. friends in the Middle East.

"What it does is it gives them another excuse to be angry at America," said former senator James G. Abourezk (D-S.D.), who represents Arab organizations in the United States. Abourezk said it is possible, however, that the recent Stinger decision will give a new opening for Arab states still interested in purchasing top-of-the-line portable missiles to protect their security interests.