WAS THAT Donald Regan, the former White House chief of staff (and first-term secretary of the Treasury), the man last seen departing the White House in a surly huff as his many detractors cheered? Yesterday's Donald Regan, testifying at the Iran-contra hearings, was a pussycat. The Democrats, yet, fell to chuckling and swapping one-liners with him and competing with each other in praise of his readiness to forgo immunity, executive privilege and counsel and to tell his tale. To the extent that these hearings amount to an exercise in purging a terrible experience by laying out some if not all of the record of disturbing events, Mr. Regan was doing his part. But the story he had to tell was in some ways like many of the others: both hard to credit and, if you did credit it, utterly depressing.

There was much irony. Here was the man known at the time as the consummate bureaucratic tough guy, someone who gained a reputation as vacuuming up the perks and prerogatives of others, who sometimes conveyed the impression of regarding himself as the top sergeant, and the president as the recruit. But by his account yesterday, he was largely out of the action. He was not involved deeply in the matters Congress is now examining. On the contrary, he claimed a place in the considerable company that was fenced out and/or deceived by the National Security Council staff people and, it seems, by theman who increasingly seems the presiding spirit of the whole Iran-contra affair, the late William Casey. Donald Regan had no shredder, he now says. Nor, it seems, as he tells it, much of anything to shred.

The trouble with the Regan testimony, which continues today is this: genial, mellow and funny as he now is, innocent of wrongdoing and impropriety as he may have been, he cannot avoid judgment for his service as chief of staff. The first responsibility of someone in that position is to protect the process of presidential decision-making -- to see to it that the president has available material he needs to make right decisions on important issues. Mr. Regan in his time was not able to provide President Reagan with the goods. Accepting his account at face value, you are compelled also to accept the notion that something rather like a palace coup had been conducted and that the man who prided himself on owning the keys to the palace did not know that the coup had occurred.