Pernell Roberts, the outspoken actor who played Adam Cartwright, the thoughtful eldest son on television’s seminal western series “Bonanza” and who was the last surviving member of the show’s original family, died Sunday at his home in Malibu, Calif. He was 81

The cause was pancreatic cancer, said his lawyer and friend Richard A. Stone.

Dark-haired, handsome and athletically built, Mr. Roberts was a rising theater actor — a busy Shakespearean, in fact — who had performed in a number of classic plays Off Broadway and on before he landed, in 1959, on the Ponderosa, the ranch that provided the setting for “Bonanza.”

Adam, the first-born son of Ben Cartwright (Lorne Greene), a three-time widower who had had a son with each of his wives, was the book-smart one; Hoss (Dan Blocker) was the comic bruiser; Little Joe (Michael Landon) was the adorable but impetuous youngster with a lot to learn. When Mr. Landon died in 1991, Mr. Roberts was the only Cartwright left. According to a statement provided by Mr. Stone, he referred to himself jokingly as “Pernell the-Last-One Roberts.”

Though “Bonanza” stayed on the air until 1973, Mr. Roberts left after the 1964-65 season. He had battled with the producers over the dearth of nonwhite actors and crew members on the show, and complained that the scripts were unchallenging for both actors and audiences. Actually, his words were harsher than that: “Junk TV,” he called the show, accusing NBC, the network that broadcast it, of “perpetuating banality and contributing to the dehumanization of the industry.”

In subsequent years, he appeared in numerous movies and touring theater productions, and made guest appearances on many television series, including “The Rockford Files,” “The Paper Chase” and “Vega$,” before landing in the title role on “Trapper John, M.D.” an hourlong medical drama that was something of a spinoff from Robert Altman’s 1970 film, “MAS*H,” which was set behind the battle lines in the Korean war.

A rollicking surgeon with a healthy disrespect for authority, Trapper John McIntyre was played by Elliott Gould in the film and Wayne Rogers in the popular television series; “Trapper John, M.D.,” which ran seven seasons, beginning in 1979, relocated him as the chief of surgery at a San Francisco hospital decades after his Army service.

Pernell Elven Roberts Jr. was born in Waycross, Ga., on May 18, 1928. He briefly attendedGeorgia Tech before joining the Marines in 1948; a musician who played baritone horn, sousaphone, tuba and percussion, he was assigned to the Marine Corps Band. On the G.I. Bill, he attended the University of Maryland, where he was drawn to acting, eventually leaving school to work at the newly formed Arena Stage in Washington. He moved to New York City in 1952.

Mr. Roberts was married four times and divorced three. A son, Christopher, died in 1989. He is survived by his wife, Eleanor Criswell.

In the burgeoning Off Broadway theater scene in New York in the mid-1950s, Mr. Roberts gained some notoriety in Shakespearean roles. He played Antonio in “Twelfth Night,” Mercutio in “Romeo and Juliet,” and the title role in “Macbeth,” a performance The New York Times critic Brooks Atkinson described as “thoroughly masculine — a ruffian who bulls his way through the plot.” In 1956, he was cast by the AmericanShakespeare Festival in Stratford, Conn., as Petruchio in “The Taming of the Shrew,” a production that reopened on Broadway the following year.

Moving to Hollywood, Mr. Roberts appeared in films that included “Desire Under the Elms,” which starred Sophia Loren and Burl Ives.

Perhaps it was his high-minded background that gave him reason to bite the hand that fed him.

“I feel I am an aristocrat in my field of endeavor,” he said in a 1965 interview with The Times. “My being part of ‘Bonanza’ was like Isaac Stern sitting in with Lawrence Welk.”