THE TEMPERATURE went down as the calm and exceedingly well-disciplined Rear Adm. John Poindexter took the stand yesterday, wearing civilian clothes and offering testimony on what he, President Reagan and others knew and when they knew it. His dispiriting answer was that they knew different things and knew them at different times. Adm. Poindexter, a man with a keen sense of compartmentalization, claims that he was himself cut off from key business being done by an ostensible subordinate, Lt. Col. Oliver North, and CIA director William Casey. On its face his testimony raised the question of how the administration operated at all.

The White House was delighted to hear Adm. Poindexter say he had not told the president of the diversion of funds from Iran to the contras. That statement, going to a point much and excessively advertised as do or die for the president personally, supported Mr. Reagan's denials that he knew of this egregious transaction. The White House was apparently less delighted to hear Mr. Poindexter report that, as the clouds lowered, he had destroyed a year-old presidential finding authorizing arms-for-hostages dealings with Iran. Mr. Reagan had denied authorizing any such trade.

In both cases, however, the White House reaction is off the main point. Adm. Poindexter gave these reasons for not telling the president about the diversion and for shredding the finding: to protect the president and afford him ''plausible deniability.'' But, as Sen. Warren Rudman observed, to do this would be to deny Mr. Reagan the opportunity to make major political choices that should have been a president's to make. The diversion went ahead, to the president's later deep embarrassment. And evidently without being reminded of the Iran finding, Mr. Reagan plunged into a series of damaging false and contradictory statements about his role. Taking Adm. Poindexter's testimony at face value, you would still have to say that "protecting" the president in his particular way, Adm. Poindexter blinded him and let him inflict upon himself repeated political wounds. The admiral's particular sense of duty -- actually a very peculiar sense, since it virtually writes off consultation with Congress -- came across strong. Still, his was a very strange and narrow idea of service to a president.

The admiral's testimony continues. More needs to be known of the dispersal of information and authority in the administration's upper reaches and in particular of the discrepancy between the five memos on the diversion that Col. North testified he sent up the chain and the one -- only one -- that Adm. Poindexter reported he received. What about the other four? And if it is true that on the fund diversion Col. North believed he was acting with presidential authorization, that Adm. Poindexter in fact did authorize him to do what he did and that the admiral was acting as he deeply believed the president wished him to, we all need to be reminded of something: what was it that Col. North was fired for, again?