Will the day come when some worn-out human keeps only his or her brain, atop a bleeping body made of electronically driven artificial parts?

The idea seems hardly possible. But a few generations ago the idea of keeping a man alive by an artifical heart seemed impossible.

"By the turn of the century, every major organ except the brain and central nervous system will have artificial replacements," Dr. William Dobelle of the Institute for Artificial Organs in New York City has predicted.

"By 1996," says Dr. Willem Kolff of the University of Utah, "a marathon runner equipped with a super-efficient artificial heart might actually be disqualified because he would have an unfair advantage."

These are the statements of optimists, though Kolff is an optimist with credentials. He invented the artificial kidney machine for life-saving kidney dialysis or blood cleansing. He put artificial hearts in animals two decades ago.

Artificial organs may never equal nature's mass-produced wonders in versatility, endurance or low unit cost of manufacture.

But in many ways we are already in "an era of spare-parts medicine," Dr. Pierre Galletti of Brown University believes. Science can now or soon will be able to replace more than 50 body parts, at least to some degree, reports Professor Larry Hench of the University of Florida.

Some 2 million to 3 million artificial or prosthetic parts are now installed in the United States every year, at a cost of more than $1 billion, says a forthcoming issue of Perspective, the national Blue Cross-Blue Shield magazine.

When a part of the body fails, in short, science no longer gives up. It at least asks, "Can it be replaced?"

This attitude has already produced some organs or parts being installed by the hundreds of thousands. Among them: artificial lenses in cataract-clouded eyes; artificial pacemakers, sewn under the skin to keep faulty hearts beating; artificial heart valves; artificial blood vessels; artificial hip joints and knees, as well as shoulder, elbow, wrist and ankle joints made of metals and plastics.

Scientists are implanting electrodes in the brains of some blind subjects to enable them to see light patterns, at least, and these experiments could lead to useful sight. Other scientists are placing electronic implants in the middle ear to give deaf subjects some hearing, although not yet enough to detect normal speech.

A chemical artificial blood can temporarily carry oxygen to the body's tissues. It has already saved the lives of Jehovah's Witnesses who shunned blood yet urgently needed surgery.

Plastic artificial skin is being used in research programs to help burn victims heal. New bionic artificial arms and legs have internal motors activated by computers to better mimic real arms and legs.

The tiny chip, the same microprocessor that gives a nimble and versatile "brain" to a wristwatch or video game, is being used in some of these devices and will be used in many more. The spare parts era, say the experts, is just beginning.