A few weeks before ordering an escalation of the Vietnam War, President Nixon matter-of-factly raised the idea of using a nuclear bomb. His national security adviser, Henry A. Kissinger, quickly dissuaded him.

Mr. Nixon's abrupt suggestion, buried in 500 hours of tapes released today at the National Archives, came after Mr. Kissinger had presented a variety of options for stepping up the war effort, among them attacking power plants and docks, in an April 25, 1972, conversation in the Executive Office Building in Washington.

''I'd rather use the nuclear bomb,'' Mr. Nixon responded.

''That, I think, would just be too much,'' Mr. Kissinger replied.

''The nuclear bomb. Does that bother you?'' Mr. Nixon asked. ''I just want you to think big.''

The following month, Mr. Nixon ordered the biggest escalation of the war since 1968.