ABC, in the mid-60's, was looking for a tentpole Western along the lines of CBS' Gunsmoke or NBC's Bonanza. What they got fell closer to the latter than the former, but didn't have the staying power of either one.
As Bonanza was set in Virginia City, Nevada, and built around the Cartwrights, The Big Valley was set in Stockton, California, and centered on the Barkleys. Victoria (Barbara Stanwyck) was a widow with three sons and a daughter, which trumped the men of the Ponderosa. Heath (Lee Majors) was sort-of the black sheep of the family, as he was allegedly an illegitmate son. Jarrod (Richard Long) was a lawyer. Nick (Peter Breck, ex-Black Saddle) was the ranch foreman. Audra (Linda Evans), the daughter, was the predictable damsel in distress. While the Cartwrights had an Asian cook, the Barkleys had an African-American butler (Napoleon Whiting) who was treated with the utmost respect. Some reference sources referred to the butler as the "major domo" who was put in charge when the family was away.
The Big Valley, a Four Star production, lasted four seasons (1965-9), and attracted an eclectic collection of guest stars. Comedian Marty Allen, for example, appeared in at least two episodes. Adam West (Batman), who'd previously worked for Four Star on The Detectives with Robert Taylor, reunited with Linda Evans, his co-star in the movie, "Mara of the Wilderness", in one episode, which I believe came after Batman ended. He wasn't the only one to swap his cape for an encounter with the Barkleys. Yvonne Craig also made a guest appearance, as you'll note in the following video, but that was before she was cast as Batgirl.
Post-Valley, Richard Long shifted to sitcoms, starring in Nanny & The Professor (1970-2) and the short-lived Thicker Than Water before his untimely passing while the latter series was in production, if memory serves. Of course, Lee Majors & Linda Evans each achieved iconic status much later. Majors in The Six Million Dollar Man, and Evans in Dynasty. It happens that they're also the only two core cast members still around.
SpudTV uploaded a season 1 open & close, taken from Encore Westerns. Currently, Me-TV & INSP share cable rights.