Dr. McCoy's handheld "medical scanners" were actually modified salt and pepper shakers, purchased originally for use in "The Man Trap", in which a character was seen using a salt shaker. They were of Scandinavian design, and on screen were not recognizable as salt shakers; so a few generic salt shakers were borrowed from the studio commissary, and the "futuristic" looking shakers became McCoy's medical instruments.
Uhura's line, "Sometimes I think if I hear the word 'frequency' again I'll cry", is drawn from an incident during filming. As early as it was in the series, Nichelle Nichols was already tiring of the repeated line, "Hailing frequency open, sir"; at one point she turned to Leonard Nimoy and told him, "If I have to say the word 'frequency' one more time, I'll blow up this goddamn panel."
Dr. Crater states that the buffalo (Bison Bison) of North America had been hunted to extinction. Though they had been nearly hunted to extinction by 1966, it is now unlikely that they will go extinct as they now number around 350,000.
This was the first episode to air on NBC but it was actually the sixth episode produced. NBC chose to air this episode first because they felt that it had more action that the pilot episode and it also featured a monster.