President Reagan, a 70-year-old who is healing like a 50-year-old, was recovering so fast yesterday that one of his doctors at George Washington University Hospital said he was "stunned."

He was in "excellent spirits" and "exceptionally good condition," said Dr. Dennis O'Leary, adding that the president probably could have put in a full day's work if he had not needed some sleep.

Many physicians compared Reagan to "a man in his 50s" in his high-spirited, quickly mending reaction to what was, at the least, a moderately serious gunshot would followed by major surgery.

But another doctor also said, "Both he and the country are fortunate everything is going fine."

The president, as healthy as he is, was also lucky Monday afternoon.

Several doctors said he was lucky his assailant used a .22-cal. pistol, what one doctor called "a lady's gun." Had his would-be assassin used a .45, the heavy slug could have torn a quickly lethal hole through his insides.

Second, he was lucky because the bullet hit a rib and was deflected. Otherwise it might have struck an organ more vital than the lung.

And last, he was lucky because he was minutes away from a major and first-class medical center. He could immediately be greeted by platoons of specialists of every kind. Then -- since he was bleeding internally and seriously from his wounded lung -- the bleeding could be halted before it became truly dangerous.

A report on his condition was given yesterday by O'Leary, George Washington clinical (medical) affairs dean, at a White House briefing.

Despite a mostly sleepless night in a busy intensive care unit -- "not a restful place," O'Leary said -- Reagan was alert and taking "almost no" pain medication for what many persons would find a painful condition.

Questioned closely about the president's abilities should a crisis arise, O'Leary said: "I think he is quite capable of making decisions and interacting with people. I wouldn't encourage him to put in an 18-hour day, but I am sure he can attend to the important matters of government today."

This assessment came just 18 hours after the shooting of the president and three of his entourage, including his press secretary, James S. Brady. Reagan was not told about the gravity of Brady's head wound and brain injury until yesterday afternoon.

"Just remember," O'Leary explained, the president was "coming out of major surgery. He'd been up all day and night," so his doctors had to decide "how much" news he should have to absorb right away.

Such news aside, the president's course read like an old movie script.

He had been rushed to the hospital Monday. Fifteen doctors crowded around him and quickly shoved a tube into his chest to drain out the air and blood that had deflated his wounded left lung. This quickly reinflated it.

X-rays and blood tests followed, and a blood transfusion to replace the 2 1/2 quarts the tests showed he had lost. Then came his three hours of surgery -- to search the chest and abdomen for other damage (none was found), to drain out more bloody fluid, to remove the crumpled bullet.

When the president was wheeled out of the operating arena at 6:20 p.m., his doctors left in place the endotracheal tube, the tube down his windpipe through which they had been anesthitizing and breathing for him.

Asked about the reports of Reagan's apparent joviality, O'Leary said it showed "a very young, very vital person. . . . He is tough in a good sense."

Another George Washington surgeon, Dr. Glenn Geelhoed, who was in the operating area, had another thought: "He's a performer. He recognizes that people are responding to him, and he responds accordingly . . . . He recognizes also, as any president should, that he is the center of attention, that people are aware of his statements and he should be considered upbeat."

Then, for age 70, there is simply that youth.

Dr. Eugene Erman, a Los Angeles chest surgeon unconnected with the case, said "Many people do well after such surgery. Some do exceptionally well. He is one. I think you have to remember that some people at 50 look like 70. Some at 70 look like 50. His doctors have said over and over that he's in good physical condition. This shows it."

Geelhoed put his "physiological age" in the "late 50s." O'Leary said, "he is a physiologically young person" who at this point -- and barring surprises -- is "doing beautifully."