Dr. Ruge, whose thick crop of gray hair topped a 6-foot-2-inch frame, was an unassuming, self-confident man capable of self-criticism. Despite his knowledge of the Constitution, he said he did not think about applying the 25th Amendment because of the frantic nature of the moment. He also said he was not sure what would have been the right time to revoke the declaration transferring power.

Dr. Ruge said that he wanted to be publicly invisible as the White House physician and declined to talk with medical reporters at the time. But Dr. Ruge later said that he had also erred in refusing interviews about these issues.

Fred F. Fielding, counsel to Mr. Reagan, has said that the president and the vice president told him they had discussed Mr. Reagan's wishes in a medical emergency, but those wishes have not been made public.

Dr. Ruge said he did not participate in any such meeting. But he said that if he had, he would have been more likely to have remembered the amendment's provision during the tense period immediately after the shooting.

At that point, he recalled in interviews, he would almost certainly have said, "Mr. President, you are going to have general anesthesia, and we had better think now about our earlier discussion of the 25th Amendment."

Dr. Ruge said he spoke out to help clarify the issue for future situations involving a president's incapacity.

Daniel August Ruge was born in Murdock, Neb., on May 13, 1917. After graduating from North Central College in Naperville, Ill., in 1939, he earned a medical degree and a doctorate in pharmacology at Northwestern University Medical School in Chicago.