By Robin Cook

Putnam. 364 pp. $17.95

In his ninth book, Robin Cook, master of such medical thrillers as "Coma" and "Mindbend," weaves his narrative around a fascinating question recently explored by real researchers: Why do humans age and ultimately die?

A research group has actually proposed that the brain produces a hormone -- sometimes called a death hormone -- that gets turned on late in life and causes a precipitous decline in bodily functions.

Salmon, for example, struggle back to the eddies of their origin to spawn, scaling falls and rapids as they go. Once they mate, they quickly and literally disintegrate. No one really knows why; presumably it has something to do with their unique physiology.

Could humans do the same thing -- cruise along through a healthy life and then literally fall apart? Scientists have searched for genes that could be turned on in old age to create substances that cause multisystem failure, but no such genes or hormones have been found. More recently, some researchers have argued that such genes are unlikely to have evolved in the first place, so the hypothesis itself has been dying a natural death. Aging, instead, appears to be a gradual winding down, a wearing out.

But not in "Mortal Fear." The Good Health Clinic, a Boston health maintenance organization, hires a free-lance researcher to explore whatever he chooses. The researcher, an expert in endocrinology, secretly stumbles on the death hormone and begins working on a way to counteract it. Stopping such a substance clearly would be worth the Nobel. The antidote is never found, but the death hormone does get put to use -- patients at Good Health begin dying mysteriously of unpredicted strokes and heart attacks after regular physical exams declare them in basically good health despite poor health habits, such as a lack of exercise, smoking, drinking and excessive weight.

The story quickly takes a terrible turn when free-lance researcher Dr. Alvin Hayes, a weird, drug-addicted, pornographic, mad scientist who makes the dramatic discovery, spontaneously bleeds to death in a vomiting torrent of red all over Dr. Jason Howard's dinner table. Howard, the protagonist and acting chief of medicine at Good Health, becomes enmeshed in a quest to understand Hayes' bizarre death and discover his breakthrough.

While the concept of a death hormone makes for a great yarn, the book suffers from breakdowns in logic and, more importantly, from cardboard characters who move without motive or simply are not believable.

Howard refuses to accept Hayes' mysterious death even though the coroner concludes the causes were natural. And even though his own patients are dropping like flies around him, Howard continues to sign their care over to other physicians while he pursues Hayes' breakthrough. He never gets a suspicion about the similarity between the rash of deaths among his own patients, Hayes' death -- which has remarkably similar features -- and the unknown discovery. To the reader, of course, it is perfectly clear.

The book also suffers a lack of edge-of-the-chair drama. When the showdown comes, Howard, despite "his Indiana Jones style fedora and his 'lived-in' Burberry trench coat," encounters "mortal fear" alone in a dark room where he has been locked to await his fate by the antagonist's hit man. All of Howard's escapes depend more on luck and serendipity than on skill or cleverness -- the arrival of the police early on, the arrival of friendly thugs in the end who save the wounded Howard from the killer's coup de grace.

Other characters defy believability. Howard's principal assistant in unraveling the plot is Hayes' former live-in lover, Carol Donner, a young, beautiful topless dancer secretly working on her PhD in psychology at Harvard. Plus, she has a muscle-bound bodyguard named Bruno supplied by her overprotective crime lord uncle who owns the topless bar. Right.

Despite the flaws, however, "Mortal Fear" is fun to read. Cook has a clean, economical writing style that holds the reader's attention and clearly explains complex science without taxing the mind. Now that he has the outline and the writing techniques down, he needs to put some flesh on his characters.

The reviewer is science editor of the Health section of The Washington Post.