William J. Schroeder, the only man in the world with a mechanical heart, today sat up in bed, dangled his legs over the side and joked with hospital staff.

Despite the patient's unique status, he's reacting "just the way you would expect an ordinary heart patient to be reacting after three days," said Dr. Robert R. Goodin, head of cardiology at Humana Heart Institute International.

Schroeder's mental and physical condition was even better than doctors dared hope. "I'm amazed -- I'll be very honest with you -- amazed he has had the strength and progress at this point after the surgery," Goodin said in his first meeting with reporters here since Schroeder's artificial heart was installed Sunday in an experimental operation.

Goodin noted that in March 1983, Schroeder, like many Americans with heart disease and chest pain, underwent double coronary bypass surgery. "He feels he is no different, no more uncomfortable" now than after the previous surgery. "That's a bit remarkable."

Goodin added that Schroeder's wife, Margaret, and his children had said he was "more comfortable . . . than he had been for months prior to the surgery."

Schroeder's diet also became more varied today -- Jello, custard, milkshakes, Popsicles, apple juice, grape juice and hot cereal -- and he may soon get the beer he requested Tuesday.

His personality began to shine through. "His spirits are excellent. He is alert and cooperative," one of his doctors observed.

Schroeder's deteriorating heart and restricted blood flow had threatened his life to the point where he qualified as a candidate for the artificial heart. Because the surgery is so experimental, recipients must have no other treatment options.

Because of his diabetes and age -- 52 -- the Jasper, Ind., retiree did not qualify for a heart transplant. With his mechanical heart, Schroeder does not face the rejection problems that transplant patients face.

Because the mechanical heart is attached to an external, 323-pound drive system that carefully regulates heart rate and blood flow, "it's hard to have heart failure," Goodin noted. Most patients who undergo heart surgery face the risk of dangerous irregularities in heart rhythm after surgery.

Because the artificial heart has been tried only once before in a human being, however, there is no way to know how well Schroeder will do in the days to come. In preparing possible candidates, Goodin said, "we have to be on the pessimistic side. We have to assume the worst and hope for the best."

Schroeder remains in critical condition in the intensive coronary care unit at Humana Hospital-Audubon, part of the for-profit company sponsoring the human research. Though Schroeder's condition was stable today, Goodin said he is considered critical because his "condition could change quickly."

The severe bleeding Schroeder experienced after his surgery appears to be less of a problem now, Goodin said.

In the next week, the medical staff will be watching for signs of infection (to which Schroeder is more vulnerable because he has diabetes) and life-threatening blood clots to the lungs or brain (a danger for all surgery patients confined to bed).

At the morning medical update, Humana heart chief Dr. Allan Lansing said Schroeder's temperature remained slightly elevated, in the 100 to 101 degrees F. range, but noted that this is common after surgery. The patient's white blood cell count -- the number of infection-fighting cells -- was down a little today, "suggesting no signs of infection," Lansing said.

Doctors have tried to counter the seizures that afflicted the first artificial-heart recipient, Barney B. Clark, in his first week of recovery by slowly increasing the flow of blood through Schroeder's artificial-heart machinery. It was later thought that the powerful equipment had been speeded up too fast in Clark's case.

Lansing said today that the cardiac output was now 4 1/2 to 5 liters of blood a minute, which is slightly below normal for healthy persons but higher than Schroeder's blood flow before the surgery.

He is receiving doses of nitroglycerin to combat high blood pressure, small amounts of an antibiotic called cephalosporin, insulin for his diabetes and heparin, a drug to prevent clotting.

While being weighed today -- an uncomfortable process much like inserting a forklift under the patient -- Schroeder said, "Boy, I'm going to remember this, starting with the big guy." He pointed to Dr. William C. DeVries, the lean, 6-foot-5-inch surgeon who implanted his mechanical heart.

Schroeder, who is about 6 feet tall and weighed 202 pounds before surgery, has gained 14 pounds. Lansing attributed this to fluids accumulated during surgery and said it did not appear to be a problem.