President Reagan was in stable condition yesterday evening at George Washington University Hospital after three hours of surgery to drain blood from the left side of his chest and remove the bullet lodged in his left lung.

A surgical team led by Dr. Benjamin Aaron and Dr. Joseph Mr. Giordano performed a thoracotomy, an operation in which the rib cage was opened and the president's heart and lungs were examined.

About two quarts of bloody fluid were drained from the left side of the chest, where they had collected between the lung and the lining of the chest cavity. The president also received a transfusion of about 2 1/2 quarts of blood.

"He certainly sailed through it," said Dr. Dennis S. O'Leary, dean for clinical affairs at George Washington University Medical School. Considering the fact that Reagan is 70 years old, O'Leary said, his response to surgery was "maybe not medically extraordinary, but just short of it. . . . . He's physiologically very young." O'Leary added: "At no time was he in any serious danger."

O'Leary said the bullet had not struck the heart or any major blood vessels with the chest. It entered the chest below the left arm, traveled slightly downward and struck the seventh rib, then ricocheted into the chest, traveling about three inches through the lower lobe of the left lung.

All the bleeding appeared to be caused by the wound made in the lung tissue, O'Leary said.

He said the bullet, severely distorted by its collision with the rib, had been removed from inside the chest. The type of bullet could not be determined immediately.

According to Michael Borowski, a technician who was present during the surgery, Giordano and another doctor first performed a minor operation called a peritoneal lavage. In the operation, fluid was put into the president's abdomen through a small incision below the navel, then drained out again to make certain that there was no bleeding from injury to abdominal organs. O'Leary said the fluid was clear, indicating that no such injury had occurred.

Next, Reagan had to be rolled form his back onto his right side and redraped for the second, more major operation, the thoracotomy. Borowski said that to perform the thoracotomy, Aaron and Dr. Kathleen Cheyney, a clinical instructor in surgery, made a six-inch horizontal incision on the left side of the chest, just under the nipple.They then used instruments to spread the ribs apart, and carefully examined the lung, heart and major blood vessels to locate the bullet and to determine the extent of the damage.

"They couldn't find the bullet at first," he said. He said they tried different tactics, finally taking an X-ray while the president was on the operating table.

He said two Secret Service agents, wearing surgical scrub suits, stood in corners throughout the surgery watching the procedure.

In his statement at about 7:30 p.m., O'Leary said the president was awake in the recovery room and was breathing normally. "He is doing extremely well," he said.

He said Regan had "a couple of chest tubes" in place, which had been inserted through the chest wall to drain blood and air and reinflate his left lung, which had collapsed.

Asked whether the lung might collapse again, requiring further surgery, he said it was "very doubtful." He said he was uncertain how long Reagan would be hospitalized, but that it might be as long as two weeks.

"I do not believe there is any permanent injury," he said.

The lung collapsed when the bullet punctured it, causing bleeding. A collapsed lung causes chest pain and shortness of breath, but is not usually fatal.

However, bleeding from the heart or major blood vessels in the lungs and chest can be fatal, so the doctors' major concern was to do exploratory surgery to make certain that those organs had not been damaged. In some cases, a collapsed lung can cause shifting of the heart and blood vessels to the opposite side of the chest, another dangerous complication.

O'Leary said the president reached the emergency room about 2:30 p.m. According to a nurses' aide who watched, Reagan walked through the hospital door supported by Secret Service agents, but collapsed to the floor once he got inside and was carried into the emergency room's trauma area. f"He looked pale," the aide said. "He looked in pain."

Doctors immediately inserted a chest tube through the skin and muscles and into the space between the lung and chest lining to drain blood that had collected and to reinflate the collapsed left lung.

The president was given oxygen and intravenous fluid, the aide said. As he was wheeled to the operating room, 30 units of O-negative blood -- a type that can be given to anyone -- was also rushed to the surgical suite. -

O'Leary said that once the chest tube had been inserted, Reagan's lung reinflated and began to function again.

However, the emergency thoracotomy that he underwent is a major operation for a man of Reagan's age. "It's a traumatic situation in a 70-year-old man," one doctor said. Even without a wound, "opening the chest in a man that age can be a problem."

The president was under general anesthesia during the operation, and his breathing was controlled mechanically be a respirator. After about an hour of surgery, doctors came out briefly to tell Nancy Reagan that the president's condition was good, according to Lyn Nofziger, and aide to Reagan. O'Leary said the surgery was over at around 6:20 p.m.

O'Leary said the president will be in moderate pain for a day or two, possibly requiring mild pain medications. He said Reagan should be alert and able to make decisions by today, although it will be two or three months before his recovery is complete.

O'Leary said that Reagan was brought to George Washington University Hospital presumably because it was the hospital closest to the shooting. He said about 40 minutes elapsed between his arrival in the emergency room and the time the operation began,. Asked whether the president might be transferred to another hospital at some point, he said that "for the next few days it would be reasonable not to move him."

Aaron, 47, who headed the team of surgeons that operated on Reagan, is the chief of cardiovascular surgery at George Washington University Hospital. He graduated from the University of Texas Medical School in Galveston, and lives in Arlington.

Giordano, 39, is a vascular surgeon and head of the hospital's trauma team. He lives in Northwest Washington and is a graduate of Jefferson Medical College in Philadelphia.