SCRANTON, PA., AUG. 22 -- As Henry G. Barr walked into the federal courthouse here today, he looked every bit the dour and methodical prosecutor he had been in his nearly 20-year legal career.

It was a proceeding the former top aide to Attorney General Dick Thornburgh had been through countless times before, the reading of criminal charges and the entering of a plea. But this time, Barr was in an uncomfortable new role: drug defendant.

Twelve days ago, in a four-count indictment that dumbfounded his friends and former colleagues at the Justice Department, Barr, 47, was charged with using cocaine repeatedly over a four-year period and lying to federal investigators to receive a top level department security clearance.

The indictment, which includes two felony counts of false statements, has made him a new national symbol of the "recreational cocaine user" that federal officials long have contended is fueling the nation's drug problems and it has embarrassed his onetime mentor, former Pennsylvania governor Thornburgh.

One of the charges alleges that Barr continued his cocaine use as late as April 1989, while he was still at the Justice Department as Thornburgh's special assistant assigned to review major criminal investigations, including drug cases.

That charge in particular has infuriated Thornburgh, who viewed it as a betrayal by a longtime associate whom he had given a succession of sensitive state and federal posts, according to the attorney general's aides.

In his first public comments on the case, Thornburgh said today in a telephone call-in show on C-SPAN that the case "clearly demonstrates we are going to pursue violators of the drug laws, regardless of their position, regardless of their status and whether or not they are with law enforcement."

Barr -- a short, mild-mannered man with graying hair and wire-rim glasses -- appeared with his lawyer Charles Scarlata before U.S. District Judge Edwin Kosik. While Barr stood silent, Scarlata entered a plea of not guilty and asked for a jury trial that could take place as early as November. It is a case that lawyers say may provide a window into a world of high-powered, white-collar professionals who used cocaine at parties, golf outings, nightclubs, and private homes in and around the state capital of Harrisburg throughout much of the 1980s.

A 15-month investigation has already netted guilty pleas by two former Pennsylvania deputy attorneys general, a prominent Harrisburg lawyer and an investment banker.

"The charges are that Barr may have been using {cocaine} once a month or once every six weeks or so -- when someone would turn him on," said one lawyer familiar with the allegations in the case. "It was casual, limited, recreational use."

But even that allegedly limited involvement has baffled Barr's friends and associates.

"My reading on him is that he was conservative in every way, socially, politically, you name it," said one associate who worked closely with Barr for years and asked not to be identified. "He was a very conventional, family-oriented, straight-laced guy and very unexciting."

"Whenever you see something like this, you look for reasons," said James West, the acting U.S. attorney in Harrisburg, and another Barr associate. "But to me, this remains absolutely shocking. Henry Barr wouldn't have been on anybody's list to get involved in something like this. He's not the kind of person that you have thought would have even experimented with narcotics. . . . He was a very quiet, conservative low-key lawyer."

A 1969 graduate of Duquesne law school, Barr served as an assistant U.S. attorney in Pittsburgh under Thornburgh in the 1970s, handling everything from bank fraud to pornography cases. In the early 1980s, he received brief statewide reknown when Thornburgh, by then governor of Pennsylvania, named him to head the state justice department's criminal division.

Barr, with West as his deputy, headed the investigation into a statewide lottery scandal and won some attention for his methodical, and successful, handling of the case and mention in the newspapers as the "straight-arrow" prosecutor.

But it was also about this time, friends say, that they began to notice subtle changes in Barr. After divorcing his first wife, Barr grew his hair longer and bought an Alfa Romeo sports car.

"I think Henry was going through something of an identity crisis," one longtime friend said.

It was also about this time, friends say, that Barr began spending more time with Pennsylvania deputy attorney general Richard L. Guida, a flamboyant local prosecutor who was known around Harrisburg as something of a bon vivant.

Guida, who left the state attorney general's office in 1986 amid allegations of cocaine use, last week pleaded guilty here to one felony count of cocaine distribution. Court papers filed by his lawyer state that Guida told the grand jury that he first used cocaine with Barr at a New Year's Eve party in 1984, continuing to use it with him, girlfriends and others on repeated occasions over the next four years.

But associates are quick to say that if Barr had a drug problem they never saw any indication of it. In 1984, then-Gov. Thornburgh named Barr first as his deputy general counsel and later as general counsel, placing him in charge of an office that oversaw more than 300 lawyers throughout state government. And when Thornburgh became U.S. attorney general in 1988, Barr was one of a half-dozen aides who accompanied him to Washington.

During the nine months he spent at the Justice Department, Barr served as Thornburgh's liaison with U.S. attorneys' offices and department agencies. Among his duties was helping Thornburgh monitor the department's escalating drug war: Barr sat in on meetings of the Reagan administration's National Drug Policy Board and helped broker turf disputes between the Federal Bureau of Investigation and the Drug Enforcement Administration.

"He seemed very committed to seeing that we get a handle on that {drug} problem," said Tony Daniels, an FBI assistant director who says he found Barr "very professional and very competent."

But according to other Thornburgh aides, Barr's tenure at the department was destined to be limited.

Barr shared a house in Alexandria with three other Thornburgh aides: administrative assistant Murray Dickman, drug policy liaison Richard Weatherbe and Barry Stern. But Barr's second wife, Paula, a former courtroom artist, was opening a custom framing shop in Harrisburg and Barr was frequently commuting there.

Dickman and executive assistant Robert Ross said Barr was not spending enough time in Washington and it was becoming increasingly clear he had to leave. "It just wasn't working out," Dickman said.

As a result, Barr left the Justice Department on May 12, 1989, and returned to a partnership at a prestigious Harrisburg law firm, a job he recently quit after his name surfaced in the cocaine probe.

In the weeks prior to the indictment, Thornburgh and five of his other staff aides were interviewed by the FBI about Barr. All denied that they ever saw any indication he was using cocaine, department officials said.

"I would remind you," department spokesman Dan Eramian said, "people who use drugs don't wear signs around their neck saying,

" 'I'm a drug user.' "