In the second-floor basement in the nuclear warhead storage-building, there is a miniature Moai Head statue in one of the rooms.

In the Japanese version of the game, the knocked out guard Spoiler:outside of Meryl's cell will still be wearing underwear. In other versions of the game, the guard is found naked with his rear-end blurred out.

There are two direct references to Hideo Kojima's previous game, Policenauts, in the game. In Hal's lab where the player first fights Gray Fox, there is a Policenauts poster on the northern wall. When talking to Otacon, he begins to express his love for Japanese animation and robots, showing footage from Policenauts as well.
In the Japanese version of the game, if the player has save data from Policenauts, Psycho Mantis will comment on it.

Shortly after defeating Grey Fox and meeting up with Hal, the player will have to find Meryl, who is currently disguised as a Genome Soldier. When the player finds her, she will run off into the women's bathroom and change out of her disguise.
If the player follows her quickly enough, the next cut-scene will show her without any pants on, as she didn't have enough time to change properly.
However, if the player goes into the bathroom before seeing Meryl and calls Mei Ling on the codec, Mei Ling will chastise Snake for going into there.

After the cut-scene inside the bathroom with Meryl, if the player goes into 1st person mode and stares at her, she will ask Snake to "stop staring at her like that." If the player continues to stare at her in 1st person mode, her skin will slowly turn redder from blushing.

If the player hits Meryl, she will retaliate with a hard slap, knocking off health. Meryl can end up killing Snake through this, and when she does the Game Over screen will have her voice saying "This can't be!"
If the player does this in the caves portion of the game and the dogs are around, she will call for the dogs to attack Snake. If the player immediately hides in the cardboard box after attacking Meryl, the dogs will instead pee on the box Snake is hiding inside of.

Metal Gear Solid borrows a good number of gameplay/plot elements from the previous game, Metal Gear 2: Solid Snake. The similarities are numerous, but the most notable ones are:
• The idea of looking on the back of the game's packaging/cd case for a hidden codec frequency. Both contact a female contact that help Snake out on his mission and both are even the same frequency, Spoiler:140.15. Both also get romantically involved.
• The idea of having to change the temperature of an object to turn it into a key.
• The idea of an anonymous source of help contacting Snake, Spoiler:both of which end up being Gray Fox.
• Snake has to run up a long flight of stairs while being chased by a number of soldiers in both games.
• The idea of a 4-man ambush inside of an elevator. The Metal Gear Solid version differs in that the four soldiers that attack are wearing stealth camouflage, which was also taken from Metal Gear 2 as there was a soldier who attacks Snake while wearing Stealth Camouflage.
• A battle with a Hind-D using Stinger Missiles.
• Snake has to battle an operational Metal Gear, and after destroying them he then has to fight the pilot of said Metal Gear in hand-to-hand, as his gear is taken away from him.

After the first time Meryl gets possessed by Psycho Mantis just outside of the Commander's Room, using the first-person perspective will reveal it has been temporarily changed to Meryl's point of view, instead of Solid Snake. Spoiler:This is a possible foreshadowing to the Psycho Mantis fight, where trying to use the first person perspective in it will actually show Psycho Mantis's point of view instead of Solid Snake's.

The psychic powers of Psycho mantis were inspired by the 1978 film The Fury. Director Hideo Kojima even told the motion designer to see the movie so he can see a scene where a young man flies with these powers.

While in the cell after the first torture session, if the player calls Naomi on the codec, she will eventually tell Snake to "place the controller against your arm" and makes the actual PlayStation controller vibrate to sooth the "pain in the arm", referencing the player having to mash the O button to keep Snake alive while he was being tortured.

After the first torture session with Ocelot, if that player runs around the cell, perform attacks inside the cell, or knocks on the cell walls, the guard will yell at Snake to be quiet.
Hiding under the bed while he is still watching you will also make him comment at Snake, saying "What the hell are you doing?", and using the Ketchup while still in his view will make him say "What are you playing around with that Ketchup for?"

Before the torture sequence, if the player had no saved often during play, Ocelot will comment on it by remarking that they would have to "travel down that long road again" and telling him to just "give up now" in order to avoid getting killed.

After the first fight with Sniper Wolf, if the player chooses to shoot and kill the rats on the platform above where Sniper Wolf originally was, they will receive codec calls from several of the teammates expressing their distaste towards Snake's actions.

The scene in which Sniper Wolf Spoiler:shoots down Meryl and continues to repeatedly shoot her in order to bait Snake out from cover is based on the same scene from the 1987 Stanley Kubrick film Full Metal Jacket.

After defeating Psycho Mantis and listening to his final words in the Japanese version of the game, Mantis will say "it feels nostalgic." In the English version, the dialogue is changed to "it feels kind of nice," although this was then changed back into "it feels nostalgic" in the remake Metal Gear Solid: The Twin Snakes.

According to Hideo Kojima, the "nostalgia" that Mantis refers to is from him sensing his own mother's feelings during childbirth, where she used the last of her strength to help him survive.

In the English localization of the game, when Ocelot and Liquid Snake discuss their demands to the president, Ocelot states their demands as including "1 billion dollars" for both the original PlayStation/PC release and the GameCube remake. In the original Japanese release of the PlayStation/PC version, he states it as "50 million dollars". However, for the Japanese release of the GameCube remake, there is no dub of the game's voice lines in Japanese, while the subtitles translated to Japanese still say "1 billion dollars". Whether or not "1 billion dollars" or "50 million dollars" is a mistake in the game's script or was changed for future releases of the game is unknown.

According to a 1998 The Playstation magazine interview with director/producer/writer Hideo Kojima & designer/artist Yoji Shinkawa, Kojima originally had no intentions of adding the cyborg ninja character design for Gray Fox to the game, but Shinkawa drew that idea "because it's cool". At first, Shinkawa thought Gray Fox should have looked like a normal enemy soldier. He had several different patterns in mind, and thought to try drawing a cyborg-ish and ninja-looking guy, of which that design came out as a result.

Despite Shinkawa wanting his design to become the main character in his own game, it didn't. Raiden, however, became a cyborg ninja in Metal Gear Solid 4: Guns of the Patriots, and later became the main character of his own game in Metal Gear Rising: Revengeance.

During the fight with Psycho Mantis, the player is intended to swap their controller to the second port to prevent Psycho Mantis from reading their mind. However, if you call Campbell after losing to Psycho Mantis enough times, Campbell and Naomi will reveal that the statues in the room are based on Mantis himself, and attacking the statues will distract him long enough to be attacked. This alternate strategy was implemented in case the second controller port of the PlayStation is damaged, although this strategy was not included in Metal Gear Solid: The Twin Snakes on the GameCube.

According to a 1998 The PlayStation magazine interview with director/producer/writer Hideo Kojima & designer/artist Yoji Shinkawa, they commented about designing the character Psycho Mantis, specifically how elements such as him wearing a gas mask because he had "no face" were "part of expanding and developing the story", according to Shinkawa. Kojima clarified that while he didn't know why Shinkawa made those choices, he was not against them being made, stating that "unexpected things like this are part of the fun".

Early concept art of Psycho Mantis by Shinkawa includes notes that read: "His body is like a preying mantis, spindly and emaciated. He wears a mask to hide his ugly face." and "Is this Psycho's secret real face?"

The design for the Xbox BIOS was pitched by freelance graphic design artist Seton Kim. Although he was fired from the job for the imagery of "Hello Kitty trapped in an alien pod" in a mock up of a memory card screen featuring the PlayStation games Hello Kitty's Cube Frenzy and Metal Gear Solid, the final BIOS would be very close to his designs.