Shortly before he was wheeled into the operating room, President Reagan looked up at his wife, Nancy, and told her: ''Honey, I forgot to duck.''

Nancy Reagan kissed her husband and then, as he was wheeled into surgery, she turned and went into a nearby room to wait out the long afternoon.

She was pale but appeared calm. Mrs. Reagan had just returned to the White House from a luncheon when she was told by the Secret Service at 2:35 P.M. that there had been a shooting at the Capital Hilton Hotel. But it was not until she reached George Washington University Hospital a few minutes later that she learned her husband had been shot.

Joins President in Room

As her limousine pulled up to the emergency entrance of the gray, cinderblock hospital on Washington Circle, she jumped out and sprinted through the doorway. Inside, Mrs. Reagan joined the President in his room while doctors prepared him for surgery. When it was time for Mr. Reagan to be wheeled down the hall, his wife held on to the handrail of his bed while doctors and Secret Service men accompanied them.