Due to a special effects explosion during the filming of this episode, William Shatner and Leonard Nimoy both suffer from tinnitus, ringing in the ears.

Vasquez Rocks in California were used to represent the Metron planet. The fort was a constructed set built near Vasquez Rocks, originally for the TV series Tales of the 77th Bengal Lancers. In the original episode, a piece of rock wall is matted in to conceal the landscape which would demonstrate that they were using the same location for the two different planets.

This episode reveals that the transporter cannot be used while the shields are raised.

Ted Cassidy would later appear on-screen as the android Ruk in "What Are Little Girls Made Of?"

Despite common belief, the episode is not based on Fredric Brown's short story "Arena," which involves a human and alien soldier both struggling to solve a lethal puzzle presented to them by another alien race, with the loser and his race being wiped out. Rather, Gene Coon came up with the idea on his own but the production crew later realized it was similar to Brown's. They got permission from Brown to use Coon's story in return for giving the author a story credit.

McCoy: I, for one, could use a good nonreconstituted meal.
Spock: Doctor, you are a sensualist.
McCoy: You bet your pointed ears I am.

Kirk: I am in personal combat with the creature the Metrons call a Gorn. Immensely strong, he has already withstood attacks from me that would have killed a human being. Fortunately though strong, he's not agile. The agility, and I hope the cleverness, is mine.

Metron: Your captain is losing his battle. We would suggest you make...
whatever memorial arrangements, if any, which are customary in your culture. We believe you have very little time left.
McCoy: We appeal to you in the name of civilization. Put a stop to this!
Metron: Your violent intent and actions demonstrate that you are not civilized.

The Gorn: Earthling! I grow weary of the chase. Wait for me. I shall be merciful, and quick!
Kirk: Like you were at Cestus III?
The Gorn: You established... an outpost in our space!
Kirk: You butchered helpless human beings!!
The Gorn: We destroyed invaders, as I shall DESTROY YOU!!!

The Metron: Sparing your helpless enemy who surely would have destroyed you, you demonstrated the advanced trait of mercy, something we hardly expected. We feel that there may be hope for your kind. Therefore you will not be destroyed. It would not be civilized.

The Metron: You are still half-savage -- but there is hope.

Kirk: We're a most promising species, Mr. Spock, as predators go. Did you know that?
Spock: I frequently have my doubts.
Kirk: I don't, not anymore. And maybe in a thousand years or so we'll be able to prove it. Never mind, Mr. Spock, it doesn't make much sense to me either.
Spock: A thousand years, Captain?
Kirk: Well that gives us a little time.

Due to bad matte work, the blue line around the Enterprise view screen wobbles noticeably throughout the episode.

In this episode, Kirk describes diamonds as "worth a fortune." A few episodes later, in "Catspaw" he will spurn Korob's offer of precious stones, saying that such things can be readily synthesized.

This episode reveals that the transporter cannot be used while the shields are raised.

This episode is the first to feature the photon torpedos referred to by name.

This is the first time that the "Federation" is referred to by that name.

What Changed in the Remastered Version
The expected cleanups of color and sound are present - the episode is crystal clear. Enhancments include the replacement of Cestus III with a CGI planet and the replacement of the Enterprise in all shots with a CGI ship. The grenade explosion at Cestus III was enhanced - the original effect was a green wash; the revised effect looked more like a photon torpedo explosion. The Gorn ship was added as a distant screen artifact; it appeared to consist of a large central body and two side projections that might be nacelles. Several displays around the bridge were enhanced. phaser fire from the Enterprise is made the standard blue as opposed to the red from the original, and launched from the proper spot on the ship unlike the original episode. The Gorn's eyes occasionally blinked, and a bit of sparkle was added in a few places. A bleeding wound appeared on the Gorn's back after Kirk rolled a rock onto it, but disappeared in later shots. The Metron was enhanced with an additional reflective effect and proximate sparkles - perhaps this is what Michael Okuda meant when he said the team planned to leave the aliens alone, but "might add some sparkle." A missed opportunity was the scenes where the Metrons permitted the Enterprise crew to view the contest - the picture inside the viewscreen jumps noticeably. Stabilization of this image would have enhanced the effect. The kaleidoscopic strobe the Metrons projected when speaking to the Enterprise crew was also left unchanged; replacement of this uninteresting effect with something better would have improved the episode.

Original vs. Remastered
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The Metron: We shall not destroy you. It would not be... civilized.

This quote from the final scene is somewhat out of place until one knows some context. In an early treatment, the Metrons planned to destoy the winner of the contest since that individual represented the race more dangerous to their own civilization. They told the combatants they would destroy the loser (and his ship) to motivate them. Kirk's act of mercy suggested that although he won, the Federation was heading in the correct moral direction and the Metrons judged that it would not be likely to threaten their civilization by the time it gained the actual ability to do so. This bit disappeared during rewrites, except for this one confusing line.