It was an arresting moment when Sen. Howell Heflin put Donald Regan through his opening catechisms during the Iran-contra hearings. Yes, Regan affirmed, he had dispensed with his Fifth Amendment rights and was not seeking limited immunity from subsequent prosecution. Then Regan volunteered, "I am also waiving executive privilege."

Just one minute. Does an ex-deputy of the president have the authority to yield such an important, if ill-defined, status? Executive privilege is, of course, nowhere mentioned in the Constitution. Usage has made it the umbrella under which the president can shelter those confidential communications believed necessary to his decision-making. With the shield of executive privilege, his White House aides, unlike his Cabinet officers, have formerly been protected from the subpoenas of congressional committees seeking to exercise "legislative oversight," also nowhere mentioned in the Constitution. A more appropriate procedure would have been for President Reagan to bestow on Regan a waiver of executive privilege.

This gets to the nub of a dilemma that goes well beyond the efforts to expose the Iran-contra travesty to the cleansing light of publicity. My concern belongs in the old baby-with-the-bath syndrome.

By his efforts to prove that he has never held a smoking gun, President Reagan has yielded substantial territory formerly thought to be within the realm of executive privilege. Throughout the hearings, we watched and listened as White House goings on were reported from the perspective of every participant except the president. Decision-making at the very top of government was stripped of all mystique. We beheld a naked docudrama of misguided and misinformed men attempting to fool each other, and especially the president, about their sordid misadventure.

Fine and dandy. The public has a right to know when the Marx Brothers take over at the White House. This right to know is separate and distinct from the right of the courts to come along at a more judicial pace and send to prison those who have committed crimes. It is also different from the powerful right of Congress to crank up the cumbersome process of impeachment when and if the president should be caught with a smoking gun.

But what about the right of executive privilege when congressional committees and subcommittees seek to probe the ordinary, garden-variety decision-making in the White House? Does Iran-contra provide a precedent? Are the PROF notes exchanged among the National Security Council staff and the penciled notes about conversations in the Oval Office subject to public scrutiny? As one who served for 4 1/2 years inside the White House, I don't believe the chief executive of any enterprise can perform his job without a degree of confidentiality as he weighs his options.

The instant post mortem in Congress, as Dean Acheson once described it, bears resemblance to the gardener's pulling up his plants each day to see if they are taking root. Acheson begs the question of how the gardener is to deal with plants when he suspects that maggots have gotten at the roots.

Congress has clearly established its right, in Jefferson's words, to conduct "the Grand Inquest." However, the difficult task is to determine how soon, how often, and by what rules and procedures the inquest should be conducted.

The British have a better way of post-morteming. Each week, the prime minister and her fellow Cabinet members are subject to grilling in the House of Common. Yet there are metes and bounds to this inquisition. Mrs. Thatcher can turn off a questioner who gets too nosy just about any time she likes.

But let a real scandal erupt in Whitehall, say, a leak of the bank rate before it is published, and the British get tough. The prime minister is obliged to appoint a Court of Inquiry composed of leading jurists. No holds are barred. Even journalists may be haled before the inquiry and obliged to reveal their sources on penalty of prison.

Suppose, instead of the marathon proceedings on Capitol Hill that threatened to wear out even the TV anchorpersons, Reagan had suspended rather than dismissed Poindexter and North, appointed the Tower Commission or perhaps a more juridical committee of inquiry, and stood back with no immunity granted? My hunch is that the facts would have been discovered much sooner. While we might have missed Ollie's televised civic lessons, public opinion would have been less polarized. And the executive power which our Constitution bestows on the president alone would have been eroded less.

The writer, president of Washington College in Chestertown, Md., was a special assistant to President Johnson.