It was believed that the Kzinti's spacesuits and ship were colored pink because director Hal Sutherland was colorblind; pink and gray looked similar to him. However, according to storyboard artist/character designer Bob Kline, color director Ervin Kaplan, who had a preference for certain colors and never listened to input from others, was to blame for this artistic choice. After the episode aired, producer D.C. Fontana apologized to writer Larry Niven.
In this episode, writer Larry Niven introduced the catlike alien race, the Kzinti, from his "Known Space" novels into the Star Trek (1966) universe. Niven was asked by Gene Roddenberry to adapt his own story 'The Soft Weapon' into a teleplay. Officially, "Kzin" is the singular form of the species, and "Kzinti" the plural and adjective form. During the interrogation, Spock tells the others to imagine eating a raw vegetable. In Larry Niven's stories, the common way to foil a Kzinti telepath was to imagine eating a carrot. When talking about the history of the Slavers, Uhura says that one race rebelled against their masters. The rebel race was named Tnuctipun in Larry Niven's stories.
There were plans to have the Kzinti return in Star Trek: Enterprise (2001), the series that takes place a century before Star Trek (1966) and The Animated Series, but the show was canceled before this could come to fruition. They were later incorporated into the Starfleet Battles gaming universe by Amarillo Design Bureau. When the game was adapted for the PC in 1999, the Kzinti were changed to the Mirak as, by that time, Star Trek's license to use the Kzinti had expired. The Kzinti made another appearance in Star Trek: Lower Decks (2020), and were mentioned in Star Trek: Picard (2020).
This was the first Star Trek production since The Cage (1966), and the last until Encounter at Farpoint (1987), in which Captain James T. Kirk does not appear, making Leonard Nimoy (Spock) and James Doohan the only actors to appear in all twenty-two episodes. The three main characters in Larry Niven original short story Soft Weapon were adapted into Spock, Sulu, and Uhura. Niven was given permission to leave Captain Kirk out of the script because he could not be easily adapted into his characters.
While Commander Scott does not appear in this episode, James Doohan voiced three other characters.
It has been suggested in several behind-the-scenes books that the Kzinti and the Caitians (to whom Lieutenant M'Ress belongs) have a common ancestor, like the Vulcans and Romulans, which would explain why both races have feline characteristics.