The show begins each time the tall, lean and bearded Johnny St. Valentine Brown Jr. steps into a witness stand and raises his right hand to be sworn in.

A D.C. police investigator for 14 years, Brown, a dapper man who drives to work in a brown 1977 E-type Jaguar, is the "resident narcotics expert" at Superior Court.

During the past three years, Brown, 40, has testified at about 1,000 narcotic-related trials, or about three to five each day.

He is often a crucial witness in a prosecutor's case. Sometimes the charge is narcotic-related robbery, assault, or murder. But, the most common trial is for possession with intent to distribute heroin, cocaine, phencyclidine (PCP), marijuana, Dilaudid or Preludin (also known as "Bam," a heroin booster).

"The average person doesn't know the particulars of street drugs," said an official in the U.S. attorney's office. The official, who asked not to be identified, said that Brown, who usually testifies last in a trial, "translates street terms for drugs and ties in all the little pieces of evidence that the prosecution throws out there during the trial. He can make or break a case."

Brown, better known by his nickname, "Jehru," said he enjoys his role as expert witness. "I'm on top of the world when I'm in the witness stand," he said after a trial recently. "It's my high. I want the jury to experience the drug trip without going through it all. I want to put them there at the drug corner. I want them to see what's happening up close. I want them to live it, smell it, taste it."

Several court officials said that his testimony is often the difference between conviction and acquittal because Brown can paint a mental picture that shows how a defendant's actions during the time he was observed and arrested by police fit the pattern that police claim is standard behavior for drug users and dealers.

"He creates theater in the courtroom," said Judge Joseph Ryan, who has presided over many trials where the flamboyant detective testified. "He has good audience contact and a dynamic personality. Jurors remember Johnny St. Valentine. His personality could be the key difference in influencing a jury."

Officials said that Brown's testimony may also lead a jury to believe that a defendant was not only using a drug--a misdemeanor offense--but, that he was selling it, too, which is a felony offense.

The distinction between the two crimes can be the difference between a one-year and 10-year stay at Lorton Reformatory.

Brown said that his main objective is to help the government win its cases. He is paid about $28,000 a year to do just that, he said.

While some prosecutors and judges laud Brown's style of courtroom testimony, he does have detractors.

In court, several defense attorneys have objected to Brown's testimony on the grounds that it is electrifying and appeals to emotions rather than intelligence.

Defense attorneys try to rock his testimony by questioning his credentials or his integrity. One attorney pointed out that Brown is not licensed as a narcotics expert or certified by a professional group, though there is no such licensing, according to police.

During a trial, Brown begins his testimony by explaining to the court why he qualifies to be heard as an expert: a degree from Howard University, where he majored in pharmacology; 2 1/2 years work as a pharmacist; about 14 years as a detective, nine of those in the morals division; seminars he's attended; books he's studied; undercover work he's done at busy drug corridors such as 11th and O streets NW, where he is known by drug dealers as "Downtown Freddie."

Brown, who grew up in Chicago, Mackinaw City, Mich., and the District, said he joined the police force after serving in the U.S. Army because he had always been curious about police work. His father was a medical doctor and his stepfather a chef.

Under questioning by attorneys, Brown tells what amount of a drug is needed to get the high that drug users seek, describes what the drug is made of, how the drug is used, the effects of the drug and how dealers sell drugs on the streets of Washington.

At a recent drug trial in D.C. Superior Court, Brown slid into the witness stand and looked over the courtroom like an eagle surveying his terrain from a comfortable mountain perch. He nodded to the jury, the defendant, the defense counsel, the prosecutor. Then he looked at the jurors with an engaging smile.

At the trial, in which two defendants were charged with possession and intent to distribute Preludin, a prosecutor asked Brown to explain how Preludin, a stimulant, is used. Brown, a captivating storyteller, accented his compelling tale with eye contact with the jurors, illustrative hand and body gestures, and an onslaught of words wrapped in soft eloquence and his resonant baritone voice.

He portrayed the actions of a drug user preparing a dose of Preludin in a cooker. Using an imaginary hypodermic syringe, he injected the drug into his own arm.

In the judgment of several judges and prosecutors, Brown's style of testifying is a compelling class act. They said that unlike most witnesses, who address the attorneys and the judge dispassionately, Brown answers a question by speaking directly to the jury.

Sometimes Brown gets carried away, some attorneys said.

Brown got so animated once in describing a drug user's behavior that a defense attorney questioned whether Brown was on drugs himself.

Moving his hands rapidly and jerking his shoulders as he spoke, Brown told the jury that "when a dude is high on Preludin, he becomes happy, euphoric, jittery. And he moves a lot."

"Like you're doing now?" said attorney Retna Pullings.

Before Brown could respond, the courtroom erupted in laughter, Brown recalled.

The courtroom ambiance can often be just as important as the facts that are stated, Brown said.

"It's a shame," he said. "But, a lot of times, it's not the evidence that wins the case. It's whether the jury likes you."