Lorne Greene, 72, the imposing Canadian-born actor who served as a surrogate father to a vast television audience for years while heading the on-screen Cartwright family in the western series "Bonanza," died of respiratory problems yesterday in Santa Monica, Calif.

A viewing fixture throughout the 1960s, "Bonanza," set on the fictional Ponderosa Ranch, became part of daily life in America. The show earned Mr. Greene a worldwide reputation and a fortune in the millions.

Mr. Greene died at 12:14 p.m., according to officials at Saint John's Hospital where his condition had fluctuated sharply since he underwent abdominal surgery Aug. 19 for a perforated ulcer. During his recovery he contracted pneumonia.

After he improved enough last weekend to be moved out of the intensive care unit his condition worsened Thursday, and he was returned to intensive care.

The actor's secretary said his wife and children were at his side when he died.

This fall he was to once again play Ben Cartwright, the patriarch of the Ponderosa, in a television movie, "Bonanza: The Next Generation."

Mr. Greene was born in Ottawa to a father who made orthopedic shoes and a mother who wanted him to become a concert violinist. After acting and directing as a student at Queen's University, he studied acting in New York for two years, then returned home to work in theater and radio. During World War II he served in the Royal Canadian Air Force.

After becoming a leading newscaster in Canada, he returned to New York to market a broadcasters' stopwatch he had invented. Picking up a television part, he went on to appear on Broadway and in a half-dozen Hollywood movies before winning the Ben Cartwright role in "Bonanza."

A production that in time drew international audiences estimated at 400 million a week, "Bonanza" made its debut in September 1959, and at first, Mr. Greene once said, it "was not a very good show."

After staying up all night, at one point, Mr. Greene said, he went to the producer and "talked about the concept, the ranch, the people on it and how they behaved."

Following that session, he said, the "show began to open up. We began to do shows about real people." Critics recognized "Bonanza" as no routine shoot'em-up, but rather a show about true human concerns.

After moving to Sunday nights in 1961, "Bonanza" rocketed to the top of the ratings and from 1964 to 1967 held the number one spot. The last show was Jan. 16, 1973.

As the model for his on-screen father, Mr. Greene said he chose his own father. "I don't know whether I could ever match my father as a person," he said, "but as an actor I try to be like him."

Mr. Greene's Ben Cartwright, vigorous in pursuit of bad men, stern in his treatment of miscreants, seemed to many to exemplify the firm side of fatherhood.

A few years ago, however, he told an interviewer that his youngest child, Gillian, was reared under the precept that "children bloom with love and wither with dissension and hate."

After "Bonanza," Mr. Greene starred in other television series, including "Battlestar Galactica."

He invested profitably in California's San Fernando Valley, worked in behalf of wildlife, made dog food commercials and once built a replica of the Ponderosa ranch house in Arizona.

While remaining a Canadian citizen, he was active for a time in Democratic politics.

"I've never gotten into a heated argument with people," he said. "Heat diffuses reason. I believe in reasoned debate."

In addition to his wife, Nancy, and their daughter, survivors include two children, Belinda and Charles, by his first wife, Rita, from whom he was divorced.