Thanks in part to the rigorous attention the Reagan administration has been giving to the sanctity of government secrets, the power of the media in the policy-making process is once again a hot item on the lecture circuit. In one such discussion here the other day, a British television producer talked quite frankly of TV as a "monster," almost inescapable, by reason of its awesome power to selectively convey violent, visual images.

Familiar exhibits were offered in evidence. The Vietnam War was lost in American living rooms. The British got the best of it in the Falklands crisis-- short-lived and by its nature not telegenic. The Israelis got the worst of it in Lebanon with nightly TV coverage of the Beirut bombardments. Government atrocities get the most play in El Salvador. Communist barbarities in Afghanistan and Kampuchea, being largely out of sight, are also out of mind.

The implication is that the "media," whether by design or sheer physical impediments, set the world agenda while distorting individual events and inflaming public passions. And there's something to it: the "media" do tend to present what is most readily accessible, which is never going to be a perfectly balanced picture.

But what we have been witnessing in recent days is the other side of the coin: the government's extraordinary capacity to define and control what is most readily accessible.

First there was the president's executive order carrying the use of lie detectors as "plumbers" for "leaks" to an unprecedented high--or low. Some say that this won't stop loose talk, and maybe it won't if we are talking about White House hatchet jobs on office- holders in, say, the Environmental Protection Agency. One doesn't readily imagine Ed Meese or Jim Baker or Michael Deaver strapped to a polygraph. But second- or third-level officials and their underlings who deal in classified materials cannot help being constrained as never before in any private conversations with news people.

The chill is already detectable. The valuable give-and-take will almost certainly be more narrowly confined. It is nonsense to suppose that this will not discourage the airing of internal dissent or that this, in turn, will not diminish public discussion and debate.

So we are getting a one-two punch: a calculated campaign to squelch publication of anything that might work against the administration's case on the issue of the moment--defense spending and arms control--and a great outpouring of self-serving presidential statements, press conferences and prime-time television addresses.

The president's command of the modern machinery of communication does not end with his ability to commandeer it almost at will. It extends to his authority to break his own rules by selectively declassifying aerial photographs and other products of U.S. intelligence available exclusively to him. Responding for the Democrats to the president's defense-spending proposals, Sen. Daniel Inouye of Hawaii raised the question of whether "the president, to bolster his views, has compromised sources of highly sensitive intelligence information."

Since the classification process is itself classified, it is hard to know. But you do know that Inouye's response--his graphs and charts of relative U.S.-Soviet nuclear power--could hardly convey the weight or authenticity of a president's prime-time display of photo-intelligence.

There is nothing new about the edge afforded by incumbency. Lyndon Johnson was not above instant declassification of secret documents to make his case for the "success" of the Vietnam war effort. Nor is there that much new about the question of whether the awesome impact of modern-day communications, with all its distortions, puts the governments of open societies at a crippling disadvantage. The administration's leak-plugging and concurrent public- relations blitz on defense spending is evidence of the equally awesome power of government to bend the media to its purposes.

What we really ought to be worried about is whether, in its efforts to redress a perceived disadvantage, the current administration may not be trampling on some of the values that its national-security policies seek to preserve..