Nancy Reagan, 64, underwent a colonoscopy, an examination to detect colon cancer, at Bethesda Naval Hospital yesterday. An aide to the first lady described the procedure as "the final part" of a general physical examination begun earlier at the White House.

Mrs. Reagan was given "a clean bill of health," said Elaine Crispen, the first lady's press secretary. "She's just fine."

Crispen said Mrs. Reagan had no specific medical complaint before the physical, but that the president and first lady have made a practice of having regular examinations.

"She says every year or two she has a complete physical," Crispen said. Yesterday, however, was the first time she had this exact procedure, Crispen said.

Mrs. Reagan spent about two hours in the military hospital, the same one where President Reagan's cancerous colon polyp was detected in an identical procedure last July, and where he underwent surgery.

Crispen said the first lady had undergone some of the tests for the routine physical at the White House medical office, but that she had been scheduled all along to go to Bethesda for the colonoscopy.

Tomorrow Mrs. Reagan resumes her official duties, presiding at a White House birthday party for a group of Foster Grandparents from around the country to celebrate the organization's 20th anniversary. Sunday she will take part in a Broadway salute to Mary Martin at New York City's Shubert Theatre, and Monday preside at her summit on drug abuse with the wives of 30 foreign leaders at the United Nations.