It ought to be an iron law that government spokesmen be there, as it was once said, at the takeoff as well as the landing. Regrettably it isn't, and too often those preparing the takeoffs stubbornly ignore the extent to which government's credibility rides on how the guy out front responds to press inquiries.

I've been there. If one can unwillfully lie, that's what I once did to the State Department press corps in response to a specific question as to whether some CIA journeymen had attempted to subvert officials of another government. The charge came from no less than the prime minister. I trusted those who assured me in writing that the allegation could be denied, and did so. I shortly learned they didn't know what they said they did. The only one at the time who knew the truth was Dean Rusk, but he was not available to me that day. He later showed me a letter of apology over the affair from John Kennedy to the prime minister. I apologized to the press corp and, after doing so, established procedures insuring that I wouldn't be humiliated nor would the government's credibility be tarnished over such incidents again. And they worked.

This came to mind while I was reading about White House spokesman Larry Speakes' problem the other day after reporters asked about rumors that American forces had gone ashore in Grenada. He flatly denied it barely 12 hours before the landing took place.

The anatomy of what occurred inside the White House sounds distressingly familiar and has been recounted in stories by Post reporters Lou Cannon and David Hoffman. Described as "furious," Mr. Speakes complained in a memo to senior White House staff that "the credibility of the Reagan administration is at stake." One hopes it reached the president.

Earlier, Mr. Speakes was told by a National Security Council official, who said he had already "knocked down" the report in response to a separate inquiry, that it could be denied. "Preposterous" is the way it was described. Mr. Speakes' source claimed he was given the denial by his superior on the NSC.

Acknowledging that he was kept in the dark about what had been planned, Mr. Speakes doesn't charge willful dissembling by his source. Which leaves open, not by much, the question whether that person was lied to. Mr. Speakes says he unwisely acted on the basis of a "narrow question," neglecting to pursue the obvious next one: if it isn't happening now, will it tomorrow? CBS correspondent Bill Plante contends that question too was denied.

Obviously things are bad when reporters know more about what's going on inside than authorized spokesmen. Some who cover the White House say this is the case all too frequently and that policy-makers appear oblivious to the consequences. The notion, which evidently runs in the upper reaches of the White House, that the press officer who doesn't know can't get you into trouble was discredited long ago. The need-to-know should include the spokesman, and if the issue demands that his response to an informed inquiry be "I won't discuss that," let it be. It may cause momentary discomfort, but it does a lot for credibility. I've been there, too.

Another denial that created needless controversy was the administration's prohibition against reporters' going to Grenada. The argument that the restriction was necessary to ensure the element of "surprise" and safety for news folks was not surprisingly met with charges of "prior restraint" from newsrooms. Forced to rely on government statements and ham radio communications, the press has been calling it the "communiqu,e war"--a characterization given to similar restraints invoked by Great Britain during its invasion of the Falkland Islands.

Preserving "surprise" may be understandable even in an invasion against wholly inferior forces. Linking that to alleged concerns for personal safety is farfetched. The smart move, while announcing the action, would have been to inform the press that arrangements for coverage would be available shortly thereafter.

So, while the story, whether reported directly or by communiqu,e, continues to subordinate what the Marines in Beirut now call "Sunday, Bloody Sunday," the debate will rage on about whether Mr. Reagan acted wisely in Grenada. He was not helped by the way it was played with the press.