Doctors treating artificial-heart recipient William J. Schroeder today reported encouraging signs that he is recovering swiftly from a major stroke in the left side of his brain.

They said he was moving the limbs on his weakened right side and again talking "intelligibly" but "a little slowly" after the Thursday night incident.

"It is a major setback to all of us, but it appears to be temporary," said Dr. Allan M. Lansing, head of Humana Heart Institute International.

Admittedly optimistic, Lansing said the stroke apparently did not cause any major permanent problems for the spunky Schroeder. Smiling, he added, "I am not the Lord. I cannot predict."

Lansing did acknowledge that doctors are worried about whether Schroeder, 52, might suffer another stroke.

The incident Thursday occurred as Schroeder was eating dinner at Humana Hospital-Audubon with his wife, and he was unconscious for about an hour, partially paralyzed and unable to talk that evening.

It was the first major complication in Schroeder's remarkable recovery since a bleeding episode on the night his aluminum-and-plastic Jarvik-7 heart was implanted 19 days ago.

Lansing described the stroke as a "slap in the face" that brought Schroeder, his family and their doctors back "to realism." Since the heart implant is a highly experimental procedure and Schroeder's medical history includes heart disease and diabetes, no one has spoken with certainty about his prospects.

Known as a man with a sense of humor and the nerve to complain to President Reagan about problems with the Social Security bureaucracy, Schroeder today showed a striking comeback, "continuing to improve throughout the day," Lansing said.

Lansing, looking tired and dressed in a green scrub outfit he wore while performing two open-heart surgeries on other patients throughout the day, began a news briefing by saying:

"The weather is beautiful this afternoon. Last night, it was miserable and raining."

During the day, Lansing said, Schroeder was able to move hands and feet and "carry on a conversation" and indicated that he wants to listen to a broadcast Saturday of a high school basketball game piped into his room from his tiny hometown of Jasper, Ind.

Schroeder was also a "little bit depressed, a little bit weepy at times . . . other times much his normal self," not an unexpected mood fluctation after his ordeal, Lansing said.

Doctors are still attempting to determine the cause of the stroke and whether the artificial heart was involved.

Given the "sudden onset and rapid recovery," Lansing said, the likely cause is a "cerebral embolism."

He speculated that a "small speck of something the size of a pin," perhaps a wandering blood clot or piece of tissue from a blood-vessel wall, became stuck in or struck an artery and temporarily interrupted blood flowing to an area deep in the brain's left side.

"His stroke was severe at the time" but affected only "a small area" of the brain, Lansing said. "I am concerned that, if it turns out it is an embolism, that it could recur."

He said the Humana team does not think the stroke was related to a malfunction in the artificial heart or its power system. But he said the heart's mechanical valves might have helped cause an embolism.

While blood generally does not adhere to smooth, manmade surfaces of much of the Jarvik-7 heart, its valves have "nooks and crannies" that might disrupt blood flow and create small clots. Such valves are also implanted in other patients with heart problems.

Lansing said that, because doctors want to proceed carefully with Schroeder, the search for the stroke's cause could take seven to 10 days.

Stroke is a general term for loss of brain function resulting from malfunctioning blood vessels and most commonly results from blockage of blood vessels carrying oxygen to the brain. It can also result from a vessel rupture that causes bleeding in the brain.

About 400,000 Americans suffer strokes each year, about 160,000 of them fatal. Government estimates suggest that two-thirds of stroke survivors may be permanently handicapped.

The degree of stroke damage depends on the part of the brain affected and the length of time blood flow is disrupted. Injuries to one side of the brain generally affect the opposite side of the body.

Lansing said that a sophisticated computer axial tomography (CAT) scan on Schroeder Thursday night ruled out hemorrhaging as the cause and showed damage to a "small but detectable" area, about the size of a fraction of his fingertip.

To test whether the problem resulted from blockage of carotid arteries in the neck, which supply blood to the brain, doctors today began ultrasound tests whose preliminary reading indicated that the arteries were "clear," Lansing said.

Lansing said the damage may have occurred because Schroeder's diabetes affected the small blood vessels in the brain. He said this cannot be determined until Schroeder can be given an arteriogram, during which a dye solution is injected into the bloodstream to visualize blood flow.

If such possibilities are ruled out, an embolism would remain the likely culprit, and doctors would essentially keep their "fingers crossed" that it does not recur, Lansing said.

Schroeder has been given a blood-thinning drug, Coumadin, to try to avert blood clots.