The episode was being rewritten as it was being shot. Cast members recalled Gene Roddenberry sitting under a tree, frantically reworking the script to keep it both under budget and within the realms of believability. As a result the filming went over schedule and took seven days instead of the usual six.
William Blackburn (a professional ice skater in real life), who played the White Rabbit, got the costume from Ice Capades for free. The claustrophobic Blackburn had a really painful time wearing it, especially as costume designer William Ware Theiss had originally sewn the Rabbit head to the suit. After nearly suffocating, Blackburn tore off the head, for which Theiss became very mad at him. Finally, they negotiated and Theiss put the head back with Velcro. Afterwards, Blackburn had no problem with the costume. He also commented that wearing the Gorn head in Arena (1967) was "even worse."
A chained tiger was brought in to appear in the episode, but never directly interacts with any of the performers. William Shatner had originally hoped to wrestle it, but was persuaded that it would not be a wise decision.
This is the only episode in which the U.S.S. Enterprise is seen orbiting a planet from right to left. The shot was deliberately reversed in post-production because the shape of the Eastern United States and the Caribbean sea could clearly be seen on the globe used as a model for the planet.
Kirk appears to address the "Angela" character as "Teller" early in the episode because the original script name for her character was "Mary Teller." It was later changed to "Angela Martine" on the set when somebody noticed that the actress had already appeared as a named character earlier in the season. (Barbara Baldavin played "Angela Martine" in Balance of Terror (1966).) In the closing credits, she is identified as "Angela."