Chekov's first name, Pavel, is established in this episode, when his love interest, Yeoman Landon, calls him "Pav."
Spock's appearance is jokingly compared to Satan in the final scene. This resemblance caused discomfort to would-be advertisers when Star Trek was first being marketed.
Spock's lightning-burned shirt was auctioned off at a science-fiction convention in 1967, the same year filming wrapped.
Walter Koenig, who plays Chekov, seems to have discarded the wig he used in his earlier episodes. Since his own hair was now long enough, it was not necessary for him to wear it anymore.
Actress Celeste Yarnall, who played Yeoman Martha London, said it took a while to film the scene where she asks how the planet's inhabitants would "do it" after Vaal is destroyed because William Shatner, Leonard Nimoy, and DeForest Kelley kept making up hilarious methods for procreating. Network censors were on set and Yarnall said they were getting upset, which made everyone laugh more.