When their cases take a bizarre twist, many Washington-area prosecutors reach for Dr. Pepper -- Dr. F. Jay Pepper, chief of psychiatry at Alexandria Hospital and blunt-spoken courtroom specialist on the so-called criminal mind.

"You want to be tactful how you say this," says Pepper, pausing for a moment of self-analysis, "but my unofficial impression is that I am probably the most active forensic psychiatrist in Northern Virginia. Probably substantially ahead of anyone else."

In a time of highly publicized insanity defenses, including that of Reagan assailant John Hinckley, Pepper belies the image of a tweedy psychiatrist helping defendants escape punishment by blurring the lines of criminal responsibility.

A former Army doctor with Vietnam combat-zone experience, Pepper sports a crew cut and holds his bantam-sized body erect. His shirt-sleeves are often rolled up. He was a staff physician during the early 1970s at St. Elizabeths Hospital, the federally owned psychiatric facility in Southeast Washington while his private practice was taking root in Alexandria.

Pepper has his critics, mostly defense lawyers who say he favors the prosecution and who contend a psychiatrist's role is to heal people, not help send them to prison. The same issues have been raised in a nationwide debate over the ethics and value of psychiatric courtroom testimony in which two opposing experts often disagree on a diagnosis.

The critics appear not to bother Pepper. "All I can say is that it seems to me that the system seems to work pretty well," he says. "I guess it's easy for me to say that because I'm usually on the side that prevails."

Pepper says he is "hard but fair." If he appears tough on crime, he says, it is because many fail to understand that a forensic psychiatrist's opinion is tailored to fit a narrow legal definition of insanity. A defendant may squawk like a chicken, walk nude down a street or bay at the moon. But if mental illness wasn't directly related to his offense, he is not insane before the law.

The standard used in the federal court system and most states is that a person is responsible for a criminal offense unless he was so impaired by mental illness that he did not know what he was doing, could not appreciate the criminality of his actions or was unable to control his behavior.

"It isn't enough that there was simply a mental illness," Pepper says. "The question was, 'What was the individual's state of mind at the time of the offense and what was the role of the mental illness in the state of mind?' We have to look at cause and effect."

Pepper's sometimes abrasive style has endeared him to many a prosecutor in the District and particularly Virginia, where successful insanity defenses are rare. In a recent trial in Alexandria, for instance, Pepper described the defendant as sane and labeled the man's psychiatric defense "concocted." That certainly pleased the prosecution, which had hired Pepper. It infuriated defense lawyer John Zwerling, whose client was convicted.

"Pepper's bold," says another Northern Virginia defense attorney who asks not to be identified. "He treads where few will, and few people challenge him."

Alexandria lawyer Yvonne Weight recalls a juvenile court case involving child abuse a few years ago in which Pepper testified against his own patient. "It was the most outrageous breach of a doctor-patient relationship I'd ever seen," says Weight. "Beyond the pale."

"I felt I had an obligation to tell the entire truth," explains Pepper. "I was using the court situation to force the mother to be aware of this case. I didn't want to make it easy for her to continue sweeping it under the rug."

In cases where sanity is at issue, Pepper stresses he is available to whichever side contacts him first. But he adds, "It's pretty well assumed in Virginia on all sides . . . that an NGI a verdict of not guilty by reason of insanity is extremely difficult to obtain . . . When a defense attorney contacts me, he probably doesn't hold out much hope in the first place that I'm going to think his client was insane."

Pepper says Virginia is part of a nationwide decline in the successful use of insanity as a defense in criminal cases. "The trend has been away from making it easy for people to be found insane," he says. "The courts have been applying their own criteria more rigorously." As a result, many cases that got through on an NGI from the mid-1950s to the early '70s no longer end in acquittal, he says.

Yet, gradual changes in law and psychiatry in the past 20 years have made the insanity defense more attractive to defense lawyers, says Dr. Alan Stone of Harvard University's faculty of law and medicine. Emphasis in both disciplines has shifted to treatment and early release.

"But more generally," Stone says, "the mood in the country, I think everywhere, is to stop understanding, to stop being psychological. The judges feel that way, the lawyers feel that way and the people feel that way."

Despite Pepper's hard-line image ("I'm told I'm well respected because I'm such a straight-shooter"), he says many Northern Virginia defense lawyers seek him out in hopes of convincing a judge or jury that a lighter sentence is called for. By contrast, "the focus in the D.C. courts has simply been on the yes or no question: 'Was the person responsible at the time of the offense?' I'm usually not brought in to deal with much more than that on a D.C. criminal case."

That, says Harvard's Stone, is a significant problem of psychiatry in the courtroom. If a suburban teen-ager is caught with cocaine, his parents may hire a psychiatrist who diagnoses an "adolescent identification problem" and recommends treatment, not jail. "That's just what the judge is looking for," Stone says. "But a kid from the ghetto -- there isn't going to be any psychiatrist."

Pepper's forensic work -- about one-third of his practice -- takes him into the shadowy nooks and crannies of the mind. Court-ordered psychiatric examinations seek to determine a defendant's competence to stand trial, whether he should be held responsible for his actions or both. The process is critical since a finding of insanity can halt further prosecution. To Pepper these cases are rare.

As a clear-cut case of legal insanity, Pepper cites a man in Southeast who had developed a "definite delusional system" and shot and paralyzed someone he believed was one of his persecutors. "What he did was a product of that delusional system," Pepper says.

Another was a man suffering from manic-depressive illness who spent money faster than it was coming in. It was, says Pepper, "An emotional derangement, or in his case it can become so serious that it affects the thought processes and judgment as well."

By contrast, Pepper says he found a woman in her 30s with a long history of alleged mental illness was responsible for a holdup using a toy pistol. "At a certain point I was asked in court to sum up what I thought of the case and I said that I believed that the girl had been a spoiled brat for 32 years and that's all. And she was convicted, by the way."

In her case, says Pepper, there were some things "that didn't quite add up." As a teen-ager she suffered from "alleged seizures" and said she would sometimes choke her sister or her mother afterward. "There's no seizure disorder that's known that would cause a person to do that," Pepper says. He then led her point by point through her alleged offense -- "what she did and who she saw and why she did it and what was said and what she was thinking." He concluded she knew "exactly what she was doing and that it was a criminal activity."

Some issues confronting Pepper seem as much philosophical as medical, revolving more around questions of free will and social responsibility than mental health. For instance, self-induced intoxication is not considered grounds for an insanity plea. "How about self-induced mental illness?" Pepper asks. If a diagnosed psychotic refuses to take prescribed medicine, is "swept back into psychosis" and commits a crime, is he criminally responsible?

"Doesn't the individual have some obligation to maintain himself?" Pepper asks rhetorically. "Let's say, for instance, a person is addicted to heroin. Well, even so, one has choices . . . The inevitable result of addiction is not that one has to commit a crime."

That and other such questions, critics say, are really moral issues best left to judges and juries. "The law has pushed that job onto medicine," says Alexandria lawyer John Grad. "Psychiatry is really a soft science. There are no hard answers."

Not so, says Pepper. "The psychiatrist is not the judge and is not the jury . . . Two chemists may disagree. Two ballistics experts may disagree. And of course, in an adversary system, the lawyers disagree. So I don't see that it's so remarkable that perhaps psychiatrists may disagree, too.

"It isn't enough merely to diagnose a mental illness. You can stick a label on almost anything. But merely to have a diagnostic label does not give a person carte blanche to do what one damn well pleases. It's not a free ticket."