At the Bergen County courthouse in this New York City suburb, a jury has been impaneled for an inquiry into a nightmare. A locally prominent surgeon stands accused of having led a double life -- dividing his time between healing and murder -- in a case in which the allegations have overtones of Jekyll and Hyde.

Mario E. Jascalevich, a 50-year-old, Argentine-born and educated medical doctor, is charged with the premeditated murders of five apparent strangers in 1965 and 1966.

The strangers were patients of other doctors at a small, private osteopathic hospital in nearby Oradell, where Jascalevich was chief surgeon.

The prosecution in his long-delayed trial will claim in its opening argument set for today that he caused the deaths by injecting the patients with large doses of a hard-to-trace drug that paralyzed the muscles they needed to breathe.

The drug is curare. Its name comes from an Indian word meaning "flying death." For centuries in South America, tribes of jungle Indians have extracted it from vines and used it to coat the tips of arrows for hunting wild game.

Doctors sometimes use curare in refined form, as an adjunct to anesthesia during surgery. In small doses, it relaxes muscles. But in large doses, relaxation becomes paralysis. Without a mechanical respirator, death occurs in minutes.

Curare does not affect consciousness. It leaves a person free to contemplate utter helplessness as he or she asphyxiates.

Jascalevich, a father of two, is accused of using the drug to kill a 4-year-old, a 70-year-old retired librarian, a 26-year-old machine operator, a 73-year-old retired public works emplopye and a 59-year-old accountant. His alleged victims are of both sexes.

Their deaths occurred over a 10-month span at a time when Jascalevich said he was using curare for surgical experiments on dying dogs at a medical college nearby.

Jascalevich made that assertion after syringes and 18, mostly empty vitals of the drug were found in his hospital locker by a suspicious junior colleague on Halloween Day 1966.

A laboratory test on one of the syringes seemed to bear him out. It showed a dog hair. But administrative officials of the medical college, where Jascalevich was an unsalaried part-time lecturer, denied knowledge of any experiments.

Jascalevich explained that he worked alone, obtaining the dogs at night by tipping janitors in the college's animal quarters. He said his experiments were designed to perfect a way of removing living tissue from the liver.

After a two-week probe the county prosecutor, with whom Jascalevich cooperated, closed the books on the case. No bodies were exhumed, the prosecutor said later, becuase medical advisers told him it was impossible to detect curare in the tissues of the dead. If curare could not be detected, the prosecutor reasoned, murder could not be alleged.

Jascalevich resigned from the hospital and went on to practice medicine elsewhere in the state.

The naturalized U.S. citizen, who came to this country in 1955, spent the next decade polishing a reputation as a skilled surgeon and publishing articles in medical journals detailing at least four advances in surgical research. By most accounts, he was seen as a respected, if somewhat aloof, member of the medical community.

Then, in 1975, two seemingly unrelated events combined to breathe new life into the old accusations.

News of an investigation into a series of suspicious deaths at a Veteran's Administration hospital in Ann Arbor, Mich., triggered memories at the Bergen County prosecutor's office. By this time, that office had a new boss. He ordered the files on the Jascalevich case resurrected for review.

At about the same time, a still-unnamed person, described only as having "post-Watergate pangs of conscience" about the handling of the original investigation, approached The New York Times. The person's tale intrigued a reporter, who pursued it and turned over new information to the prosecutor.

The prosecutor decided to reopen the case. The news of this development and a recap of the then 10-year-old events appeared in The Times in January, 1976.

In the story, which was the first publicity the case received, the paper chose not to identify the surgeon who had not been charged with a crime. Instead, it referred to him as Dr. X -- a name that has become the unofficial title of the case.

In the reopened probe, the prosecutor obtained court orders to exhume the bodies of the five patients who had died suddenly, usually during the course of what was seen as normal recuperation from surgery.

Five months after exhumations, and after much laboratory testing of tissues taken from the dead, a grand jury indicted Jascalevich in the five deaths. Jascalevich pleaded innocent to murder charges but voluntarily surrendered his medical license pending the outcome of the case. He has remained free on $150,000 bail.

Much of the testimony at his trial, expected to last two to four months, will be highly scientific as the prosecution seeks to prove that the patients did not die of the natural, if sometimes unexplained, causes that their death certificates siad. The defense is expected to produce scientific experts of its own to oppose the prosecution's contentions.

All of the evidence against Jascalevich is said to be circumstantial. No motive has ever been formally alleged. Through a lawyer, the defendant once said that he thought he was being framed.