First film of a three-picture contract that Charles Bronson had with producer Dino De Laurentiis. The second and third were The Stone Killer (1973) and The White Buffalo (1977). Bronson's earlier film for de Laurentiis, The Valachi Papers (1972) was a single-picture contract. During the 1960s, Bronson had also appeared in Battle of the Bulge (1965) for de Laurentiis.
The Indian characters were played by local Spanish gypsies.
"The Valdez Horses", the novel on which this film is based, was written by Lee Hoffman and published in 1967. It won the Western Writers of America Spur Award that year.
Although set in the US state of New Mexico, the film was filmed in Almería, Andalucía, Spain.