President Reagan says there isn't "an iota of truth" to an account that aides questioned whether he was mentally fit to serve at the height of the Iran-Contra crisis. But a former aide said Friday it would have been irresponsible for him not to raise the issue.

"There's no truth to any of this," Reagan said Thursday when asked about the report in a new book that aides weighed whether to invoke the 25th Amendment, providing for the vice president to take over if a president is unable to discharge his duties."It's just ridiculous," Reagan said while attending the annual White House picnic for members of Congress. "There isn't an iota of truth in the whole story."

As to whether people were considering invoking the 25th Amendment, the president said, "I never knew that anyone was. I don't think anyone was."

James Cannon, a former Senate aide to Howard H. Baker Jr. who was on the transition team when Baker succeeded Donald T. Regan as White House chief of staff, confirmed most of the account, attributed to him, in the book "Landslide" by reporters Jane Mayer of the Wall Street Journal and Doyle McManus of the Los Angeles Times, published by Houghton Mifflin.

Cannon, in an interview on "CBS This Morning," defended his decision to raise the issue of the president's mental state and the possible use of the 25th Amendment to Baker after he had interviewed White House staff members as part of the transition.

"I think it would have been irresponsible not to given what I had been told by senior staff members who had been working with him (Reagan)," Cannon said.

Cannon said he interviewed White House aides on Feb. 27 and 28, 1987, just after Baker accepted the job, and "a pattern of responses evolved that the real problem you have to worry about is the president. They would say he's not paying attention, he's not reading his memos, he doesn't come to the office, he stays over in the residence."

On March 1, the book says, Cannon wrote a memo to Baker suggesting that his first priority be to "consider the possibility that section four of the 25th Amendment might be applied."