IT WAS a subdued and unexpansive Caspar Weinberger who told congressional investigators Friday of his frustrated efforts to block the Reagan administration's feckless Iran initiative. His calm as a witness, however, cast in stark relief his evident rage as a policy adviser who did not simply lose an argument, which as a good soldier he was prepared to live with, but who found himself cut off from crucial information about the dealings that were going forward. He discovered the White House trying to block his access to his own department's intelligence and learned of the McFarlane mission to Iran only from a foreign government.

Like Secretary of State George Shultz, who also opposed the NSC staff's -- and William Casey's -- Iran-contra policy, the defense secretary came before the committee as someone with no suspected impropriety or potentially criminal conduct to explain. Investigators and spectators instead focused on how it was that an adviser of practically unmatched personal and ideological affinity to the president could have been overruled and then kept in the dark, even as he doggedly kept the arms flowing from Pentagon stocks to facilitate a policy he thought was ''absurd.'' And on how Mr. Weinberger, as angry as he described himself to be, could have been fenced out and could -- like Secretary Shultz -- have failed to use his high standing and his direct access to the president to press his case in person.

Still the Reagan loyalist, Mr. Weinberger ascribes to ''people with their own agenda'' the responsibility for keeping the president from hearing any further dissenting word after Iran-policy discussions in December 1985 and January 1986. Testimony from Adm. John Poindexter and others confirms that Mr. Reagan was in the hands of advisers who had developed a personal commitment to the policy and were prepared to pursue it without bureaucratic scruple.

Still, Mr. Weinberger's is a very charitable explanation of the president's detachment. The heart of this affair remains that Mr. Reagan, knowing Mr. Shultz and Mr. Weinberger violently objected, gave himself over exclusively and uncritically to other voices. Mr. Weinberger's testimony continues on Monday, but only one person can go to this essential point: President Reagan.