On his 133rd day with a plastic pump pulsing in his chest, William J. Schroeder is to leave the hospital Saturday and start living more or less on his own.

"He knows he's leaving and he's excited," a hospital spokesman said today.

After becoming the first patient with an artificial heart to be discharged from a hospital, Schroeder and his wife, Margaret, are to be driven across the street to a specially equipped "transitional" apartment owned by Humana Inc., the hospital firm.

He is to be cared for round-the-clock by licensed practical nurses and registered nurses. Security guards are to keep out the curious.

If the machinery running the heart fails, a high-pitched signal is to alert Margaret Schroeder, who can switch manually to a backup unit. She also is to be provided with a "panic button" that automatically telephones the hospital, and she can talk on the phone without holding it.

When Schroeder goes for a walk or a ride in a customized van, a hospital technician is to help him get hooked up to a portable power unit.

The Schroeders hope to return eventually to their hometown of Jasper, Ind., 90 miles away.

This first milestone has been a long time coming. The move was postponed repeatedly by setbacks during Schroeder's uneven recovery, including a stroke in December and a mysterious, lingering fever in February.

By last week, Schroeder, 53, was healthy enough to leave the hospital but had to wait several days because his new private-duty nurses hadn't finished their special training, according to his surgeon, Dr. William C. DeVries.

Private nurses are needed because -- as officials at Humana Hospital Audubon belatedly discovered -- zoning regulations prohibit the hospital's nursing staff from tending Schroeder away from the hospital.

In recent weeks Schroeder's strength has returned slowly after the prolonged fever that left him weak and listless. His speech, severely impaired by the stroke, has improved unexpectedly, DeVries said.

DeVries was particularly encouraged after posing with Schroeder recently for a Life magazine cover story. The famous patient leaned close to his doctor and, according to DeVries, cracked an unprintable joke that made them both laugh.

Schroeder has been receiving most of his nourishment through a tube in his nose, but a Humana spokesman said he is beginning to eat solid food.

The first days after Schroeder's operation were marked by rapid progress and high spirits, including a spunky phone conversation with President Reagan. But 18 days after his diseased natural heart was replaced with the Jarvik-7 artificial implant, Schroeder suffered a stroke, which left him with short-term memory loss, difficulty speaking, weakness and depression. He later had a few seizure-like episodes, which his doctor described as "brief periods of disorientation."

Schroeder slowly grew stronger, walking several hundred yards a day and riding half an hour a day on an exercise bike. The medical team began planning to release him on his birthday, Feb. 14, but he developed flu-like symptoms and a fluctuating fever that reached 105 degrees -- possibly a reaction to antiseizure medication, according to Dr. Allan Lansing, chairman of Humana Heart Institute International.

The illness left the patient so weak and discouraged that Lansing worried aloud that Schroeder might never leave the hospital. Hours after Lansing's comments, however, the fever seemed to subside and Schroeder improved dramatically. The next day Schroeder took his first wheelchair ride outdoors, in the hospital parking lot.

Schroeder remained too feeble to attend his son's wedding in Jasper on March 16. That event had been a personal goal for Schroeder and one of the reasons he agreed to undergo the experimental operation.

But that day did mark another milestone for Schroeder: On March 16, he had lived with an artificial heart for 112 days -- as long as the world's first artificial-heart recipient, Seattle dentist Barney B. Clark. Clark died March 23, 1983.

The only other recipient of a permanent artificial heart, Murray P. Haydon, has been slow to recover from his surgery because he was extremely malnourished at the time of his Feb. 17 operation.

About two weeks after his heart implant, Haydon underwent a second open-heart operation to stem bleeding that had failed to stop after a monitoring tube was removed from his chest. Doctors blamed the bleeding on blood-thinning drugs that had been given in an attempt to prevent a stroke. Last month, Haydon was moved from intensive care and placed in a regular hospital room next to Schroeder's. But Haydon was returned to intensive care after developing "acute respiratory distress syndrome," a thickening of the lungs that impairs breathing. He is being weaned from the respirator that has assisted his breathing for more than two weeks.