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The working title of this film was Darling, I Am Growing Younger. Before and after the film's above-title cast credits, there is a shot of Cary Grant coming out of a house, and an offscreen voice tells him "Not yet, Cary." Grant then goes back inside the house. According to a modern source, the voice is that of director Howard Hawks. According to a June 30, 1951 Los Angeles Examiner news item, Danny Kaye was originally set to star in the picture, and a modern source reports that Hawks considered Ava Gardner for the female lead. Although Hollywood Reporter news items include the following actors in the cast, their appearance in the completed picture has not been confirmed: Charlotte Austin, Pauline Garon, Pat Combs, Perk Lazelle, Lorelei Vitek, Rita Leonard and Virgil Johansen, who served as Charles Coburn's stand-in. Hollywood Reporter news items also include Mary Field and Frank Ferguson in the cast, but they do not appear in the finished film.
According to information in the MPAA/PCA file at the AMPAS Library, the Breen Office rejected a February 6, 1952 version of the screenplay, stating: "the reason for this unacceptability lies in the fact that the story of Dr. Fulton's youth formula amounts to a story of the invention of an aphrodisiac, which mainly exploits the lurid, or what might be called 'sexsational' aspects of such a drug." After meeting with Hawks and producer Sol C. Siegel, the PCA decreed that the script would be acceptable if the nature of the youth formula was changed, including changing its name from "Cupidone" to a "pseudo-scientific type of vitamin," and if "Oliver Oxly's" sexual interest in "Lois Laurel" was made less overt. The revised screenplay was approved in mid-March 1952.
Although numerous contemporary sources refer to Henri Letondal's character as "Siegfried Kitzel," he is called "Dr. Jerome Lenton" in the film. Hugh Marlowe's character is listed as "Harvey Entwhistle" by contemporary sources, but he is called "Hank" in the picture. According to a modern source, Grant sued Twentieth Century-Fox in the early 1970s after the studio used clips from Monkey Business in a documentary about Marilyn Monroe. Grant won the case and was awarded ten dollars in damages. Monkey Business marked the fifth and final collaboration between Grant and Hawks.