Jack London's original novel was left markedly incomplete at the time of his death, and it was not until many decades later that the thriller-writer Robert L. Fish (also known as Robert L. Pike) finished it for publication, amidst much publicity. The novel is noticeably more serious in tone than this movie, although a New York Times review at the time called it "delightfully ridiculous."
Filming occurred about a year prior to release. According to lead actress Diana Rigg, the delay was due to the "rather awkward - for America - title" and Paramount's hesitancy on how to market it.
Jess Conrad, because he plays an Italian, is dubbed by Robert Rietty. The latter also dubs at least four others (unbilled actors in bit parts) as well as appearing on-screen in the role of an Italian police officer.
The film was initially announced in 1966 by United Artists with Burt Lancaster in the cast. After he dropped out, Paramount took over the film.