Originally the show was not going to include Nichelle Nichols, George Takei, and Walter Koenig due to budget considerations. However, when Leonard Nimoy learned about this, he refused to join the cast unless his friends were included. Rather than lose the most popular cast member, Filmation agreed to sign on Nichols and Takei. While Koenig could not be included because of the budget, he wrote one episode, "The Infinite Vulcan."
The episode "The Practical Joker" is essentially the origin of the holodeck.
When story editor D.C. Fontana was writing her episode "Yesteryear," she ran into opposition from the network executives when they learned her story would have Spock's beloved pet mortally wounded and requiring euthanasia. The executives were concerned that it would upset young viewers and insisted that the pet be allowed to live. Gene Roddenberry supported Fontana and reassured the management that she would handle the story appropriately. As it turns out, there were never any complaints about the episode and the story is considered the finest of the series.
The show's final episode, "Counter-Clock Incident," introduced the very first captain of the Enterprise, Robert April. Due to Gene Roddenberry's later request that the animated series not be considered canon, however, it has never been made part of official Star Trek continuity that Capt. April was the first captain of the USS Enterprise.
Many of the episodes were written by veterans of the original live-action TV series.
The life-support belts came about simply because the bulky spacesuits created for "Star Trek" (1966) were too complex to draw. The concept of the transporter came about the same way: it got the crew to the planet without the expense of filming a landing sequence every week. (Ironically, the belts were never adapted for the later live-action movies and TV series because making the actors "glow" via special effects would have cost more than making spacesuits!)
Sci-fi author Peter David later integrated the characters of M'Ress and Arex into his book series "Star Trek: New Frontier," starting with the novel "Gateways #5: Cold Wars."
One episode of this series revealed that Kirk's middle name is Tiberius.
Between "Star Trek" (1966), "Star Trek" (1973) and the six original series 'Star Trek' movies, the Season One episode 'The Slaver weapon' is the only time after his character was first introduced that William Shatner (Captain James Tiberius Kirk) does not appear. He also did not appear in the first "Star Trek" (1966) pilot, 'The Cage'.
Walter Koenig (Ensign Pavel Chekov) was the only original "Star Trek" (1966) cast member to never appear on the series.
The events of the animated series are said to take place during the final year of the Enterprise's five-year mission.
In the episode, "Slaver Weapon", writer Larry Niven introduced the cat-like alien race, the Kzinti, from his "Known Space" novels into the "Star Trek" universe.
Walter Koenig, who wrote the episode "The Infinite Vulcan," became the first "Star Trek" (1966) actor to ever write a Star Trek story. Over the following decades, many Trek actors would write films, novels and comic books based upon Star Trek, and many more would direct TV episodes and movies.
Gene Roddenberry decided that this animated series was not "canon" (as the live-action series movies are) because he did the show for the money, and he wouldn't have let the writers do some of things they did if he knew Star Trek would return in live-action.
The "Star Trek Crews" from all the Star Trek series were ranked #2 in TV Guide's list of the "25 Greatest Sci-Fi Legends" (1 August 2004 issue).
When the animated Star Trek premiered on September 8, 1973, stations in Southern California ran a different first episode than that seen in the rest of the country. Actor George Takei (Sulu) was running for a local political office and area stations were afraid that running an episode in which Sulu appeared would require them to give "equal" air time to Takei's opponents under the "fairness" doctrine then enforced by the FCC. Instead, they ran an episode in which Sulu did not appear.
Because of the series' low budget, Nichelle Nichols (Uhura), James Doohan (Mr. Scott), and Majel Barrett (Nurse Chapel) all voiced many of the extra crewmen, aliens, and "guest" characters that appeared on the series.
Filmation rotoscoped three live action USS Enterprise shots from the original show to use as stock animation. The shots rotoscoped were the ones where the Enterprise is coming toward camera in orbit (used in most of the original shows), the shot where the camera zooms in on the top of the Enterprise (where the bridge is) seen in full on "Star Trek" The Cage (1966) and a shot of the Enterprise zooming toward camera which also came from "The Cage" and was used rarely as stock footage.
Leonard Nimoy (Spock) and James Doohan (Scotty) are the only actors to appear in all 22 episodes of the series.