Most television viewers know Pernell Roberts as Trapper John McIntyre on CBS' long-running Sunday night hospital saga. Some recall he portrayed Adam, the eldest son in yesteryear's socko western series, "Bonanza."

Few know much else about Roberts, about whom little has been written in the last decade, mostly because he has become a private person who disdains interviews.

Behind the mask Roberts wears for the public is an actor who has enjoyed more than two decades of success in series television and whose professional roots are found in the Washington area.

An old hand on the 20th Century-Fox Television lot said of Roberts' shyness that he "never likes what he sees written about him. He's a lot like James Arness," the star of "Gunsmoke" who also avoided the press. They are sort of contemporary Greta Garbos, who "want to be alone."

From some of those who work with him comes the view that "he is a very humanistic and very liberal person who has tired of reading about his leaving 'Bonanza' more than 20 years ago." The show ran from 1959 to 1973. Roberts quit after six years, noting he "didn't leave the show in pursuit of an particular goal. I left for my own good."

Roberts came to the Washington area in 1946 after growing up in Waycross, Ga., gateway to the Okeefenokee Swamp. He went to Georgia Tech briefly to study engineering and worked for the Atlantic Coast Railroad as an apprentice mechanic before serving two years in the Marines.

That stint brought him to Quantico where he played a tuba in the band. He once marched in the Winchester, Va., Apple Blossom Festival parade. The next time he visited Winchester and its festival was in 1963. This time he left the tuba behind and rode at the head of the parade as its grand marshal.

He went from Quantico to the University of Maryland, where he won leading roles in four University Theater productions before leaving less than two years later. Prof. Rudy Pugliese, who directed Maryland's theater group for 20 years, said Roberts' success came as no surprise to him.

"He had it at that time. He had that magnificent voice, a great build and chutzpah. Nothing could stop him. No one could put him down," Pugliese recalled. After roles that included classics "Othello," "Androcles and the Lion" and "Antigone," Roberts decided to strike out professionally. Pugliese noted he made no effort to stop Roberts because "I knew he was ready and that he would make it. "

He went on to Arena Stage, then in its infancy and located in the old Hippodrome burlesque theater on New York Avenue NW, near Ninth Street. In his two years there, he starred in 18 productions and also was in a number of offerings at the Olney, Md., summer theater.

Then it was off to New York where he first appeared in one-act operas and ballets with the North American Lyric Threater, and as Mephistopheles in "Dr. Faustus" at the Equity Library Theater. Between shows in New York he once took a job as a cameraman for a Baltimore TV station but returned to New York with the Shakespearewrights. His performances in "Macbeth" and "Romeo and Juliet" won him a Drama Desk Award as the Best Actor Off-Broadway in 1955.

He also won enough recognition to get Broadway roles in "The Lovers" with Joanne Woodward, "Clearing in the Woods," and "Tonight in Samarkand," as well as a stay with the repertory Shakespeare Festival in Stratford, Conn.

In 1957 he was off to California and the movies. His first three were "Desire Under the Elms," "The Sheepman," and "Ride Lonesome."

Then came "Bonanza." After six years he had little good to say about his role as the serious, introspective son. He said he had no ax to grind. He believed "Bonanza" was a fine idea for the other members of the cast because they enjoyed it.

"I was in the series for six years. I fought with the powers about the scripts, character development and other things. It got so I was upset the minute I arrived for work. That's no way to live," he explained a year after leaving the show. "I don't have the psychological stamina to last under artistic compromises over a prolonged period of time. I get hostile and vindictive. It wears me down."

Two years before he left the show, he told Lawrence B. Laurent of The Washington Post, "Isn't it just a bit silly for three adult males to get Father's permission for everything they do? . . . I have an impotent role. Everywhere I turn, there's the father image."

He didn't listen to "Father" and gave up the rare opportunity for a young actor to become a multi-millionaire, even though Lorne Greene, who played his dad, advised him to "stay with the series a few more years and you'll be able to build your own theater. And you will be able to hire Tennessee Williams -- or anyone else -- to write a play for you."

Between "Bonanza" and "Trapper John, M.D." he honed many of his talents in a diversified career that included seminars on poetry, acting and stage production at universities across the country.

There were more movies and TV guest appearances, but more notably Roberts kept his hand in stage work. He toured six cities with Ingrid Bergman in George Bernard Shaw's "Captain Brassbound's Conversion," including a three-week stop at the Kennedy Center in March 1972.

Leading roles in "Camelot," "The King and I," "The Music Man," and "Try Alice" were among his stage credits at the time, as well as playing Rhett Butler in a musical version of "Gone With the Wind." He also made two TV movies, "High Noon, Part II," and "The Return of Will Kane."

Then came the role of Trapper, the doctor who had served in Korea in the "M*A*S*H" series. oberts played him as the chief surgeon at San Francisco Memorial Hospital. "When the show began no one predicted it would last long. The critics were never warm to us," said executive producer Don Brinkley.

"Trapper John, M.D." is wrapping up its seventh season (yup, one year longer than Roberts stayed with "Bonanza"). The series went on hiatus in January but is now back with eight new shows.

During the mid-January winter press tour in Los Angeles, CBS Entertainment President B. Donald (Bud) Grant strongly hinted that "Trapper John, M.D." is gonzo for good after this season.

Brinkley disagrees, and the network's official position is that no decision about next fall has been made as yet. One of the eight new episodes was shown on a Tuesday in late February. The others are due in March, tentatively scheduled to air right through the re-run season. At that time, Brinkley insisted, a decision will be made on next year. "We're not dead yet. Maybe tottering a bit. After seven years you can't help but totter a little.

"Actually we've done very well. We have some scripts we'd match up against anyone. We have a romanticized view of a hospital with a touch of irreverence, and from the ratings through the years, the poeple like it." Roberts was nominated for an Emmy following the second season.

Brinkley added, "Roberts is a fine person to work with. He's very professional and a very good actor. We've had our differences from time to time. We had some different ideas on how a scene might go, but we've been able to sit down and talk them out. Compromise, amicably."

Despite their seven years of association, Brinkley said he knows little of Roberts' private life. "He keeps to himself. I don't know what he does once he leaves the set. I do know that he's driven a red motorcycle to work at times to beat the Ocean Highway traffic." (He tooled around on a motorcycle as a Maryland student.)

Roberts lives in Malibu with his wife, Cara Knack. He has a grown son from a previous marriage. His pastimes, according to CBS (which reported that his publicity files "were empty"), are jogging, racquetball, tennis, bicycling, ice and roller skating, skiing, kite flying and playing guitar. Last year he was named chairman and national spokesman for STEPS (Steps to End Paralysis).

What will Roberts do if the show does not return next fall? "He hasn't talked to me about what he'd like to do," Brinkley said. "Not a word."