MGS1 Secrets

Technical demonstration mode

Successfully complete the following modes in order: "Training", "Time Trial", "Gun Shooting", "Survival". Note: Completing each mode will unlock the next mode in the sequence. Then, technical demonstration mode will be unlocked. Use it to watch the CPU demonstrate its best time with the gun.

New textures

Successfully complete the game two times using the same saved game file, and getting a different ending both times by getting the bandana and stealth camouflage. Then during the next (third) time the game is played, reach Ninja without using the bandana or stealth camouflage. If done correctly, Ninja will be red. There will now be new textures for Snake and Ninja and new bonus music during the ending.

Codec

If you keep calling Mei Ling and choose "DO NOT SAVE", she will tell you to stop calling her on the third try. On the fourth call, she will say nothing and prompt you to save or not. Choose not to save again. On the fifth call, she will stick her tongue out and close the channel without prompting to save.
If you keep calling Nastasha Romanenko, she will eventually tell you about her past, and what happened to her parents, which led to her hate for nuclear weapons and of war.
Every time you finish a boss battle or complete a significant event, some members of your support crew (via codec) will reveal their thoughts on the matter.
Calling people while holding some items will drum up an interesting conversation.
Almost everyone lectures Snake while he has his cigarettes equipped.
Calling Campbell when you acquire any kind of weapon will prompt him to on how to use the weapon. If you haven't already called Nastasha, he will tell you Nastasha's frequency.
Call Campbell four times during the Psycho Mantis battle. On the fourth call, he will tell you the secret to beating Mantis.
During boss fights, different characters will give you tips and information.
Call Campbell and Naomi will give a detailed background on the current FOXhound member.
Shooting the rats near the Comm Towers will prompt different responses from Snake's teammates.
Killing the ravens before the boss fight with Raven will provoke the same kind of responses; Master Miller will also correct Snake who calls them crows. Additionally, Raven will be given an extra line when he meets Snake.
All references to the "crows" will also be changed to "ravens" if Snake calls Miller first.
Call Master Miller while walking on a noisy floor. In the PSX version, he tells Snake to try "fitting his socks over his shoes" after Snake expresses his frustration trying to walk stealthily. The line is changed in the remake, however.
If you wait a while to throw away Ocelot's Time Bomb, Deepthroat will call Snake and warn him about the bomb in his equipment.
In the Japanese version, there are two hidden codec frequencies: 140.07 and 140.66. 140.66 plays classic Metal Gear themes, including Discovery, Zanzibar Breeze, and Theme of Solid Snake.

Alternate costume

Successfully complete the game two times using the same saved game file, and getting a different ending both times by getting the bandana and stealth camouflage. Then during the next (third) time the game is played, Snake will change into a tuxedo in the elevator.
Alternate title screen

Alternate tittle screen

At the title screen, press the D-pad to change the background color.

Konami game surprise

Play the game using a memory card that contains a saved game from any prior Konami title (Castlevania, ISS Soccer, Silent Hill, Suikoden, etc.). Then when Psycho Mantis attempts to read your character's mind, he will mention various game titles

Hal's Lab

In the hallway of Nuclear Storage B2, Otacon seems to have scrawled "Hal's Lab" on the door leading to his laboratory. This may be a reference to Hal Labs, where Kojima's close friend and game designer Masahiro Sakurai formerly worked.
Inside "Hal's Lab", a first generation Playstation console with a Dualshock controller can be seen in the middle part of the lab. For the remake, it is replaced with a Gamecube and a Wavebird controller. Statuettes of Mario and Yoshi can also be seen sitting on the console in the upper-right corner.

Meryl

Near the beginning of the game, when entering the air-vent to meet up with the DARPA chief, look into the vent before the chief's to get a first glimpse at Meryl doing sit-ups. If you leave and re-enter, she'll be doing leg stretches and push-ups. On the fourth time, Meryl will have taken her trousers off and repeat the same exercises in her panties.

Pantless Meryl

Follow Meryl into the Women's bathroom and reach the last stall in under five seconds. Meryl will be caught with her pants down during the next sequence.

Humorous Meryl comments

Look at Meryl with the scope or camera.

Box destinations

Equip a box while in the trucks to travel to the destination on the boxes.

Annoy Cambell

Snipe the rats above where you first encounter Wolf or Snipe the ravens when at your second encounter with Raven.

Defeating Psycho Mantis

Plug the controller into port two and press any button before fighting Psycho Mantis. The word "Hideo" will appear in the corner of a black screen. Use controller two to fight against Mantis to keep him from knowing your moves.

Seeing through Psycho Mantis' eyes

When fighting Psycho Mantis, press Triangle to see through his eyes.

Easier driving scene

To get a better view when shooting at Liquid Snake in the driving scene, hold Triangle while shooting the gun.

Mono TV?

If you call Campbell during the Hind D fight, one piece of advice that he'll give you is to listen for the sound of the rotor blades. If the player does not own a stereo television or have the sound settings on Monural, Campbell, Naomi and Mei Ling will express pity towards Snake for not owning a stereo TV set. However, Campbell reassures Snake that he can still beat the Hind.

Ghosts

Take a picture of the following locations using the camera to view hidden ghosts who are KCEJ development team members.

Name
Location

Fujimura
Elevator in the Comm Tower B complex

Fukushima
Heliport; looking out to sea from cliff

Hirano
Elevator in Comm Tower B; deep in the shaft of top level

Ishiyama
Heliport top of building

Ito
Inside elevator to tank hangar

Jerem Blaustein
Sniper Wolf's corpse

Kaneda
The mirror located in the Women's restroom

Kimura
MG underground base; tip of Metal Gear 's railgun

Kinbara
Dark area of stairs

Kitao
Decoy Octopus (fake DARPA chief) corpse

Kobayashi
Rock in canyon

Kobayashi
Johnny Sasaki holding cell, near DARPA chief

Kojima
Otacon lab; the picture frame to the right

Korekado
Men's restroom

Kozyou
Behind the watertank-like structure in the Canyon

Kutome
Observation room

Makimura
Hidden armory store room; behind the weakened wall

Matsuhana
Hallway of corpses; outside Otacons lab

Mizutani
When fighting Metal Gear

Mori
Lowest point of elevator; Comm Tower B

Mukaide
Reflection in wolfdog cave puddle

Muraoka
Water in cargo dock

Nakamura
In Meryl's blood pool; where she is sniped

Negishi
Sewage waterfall

Nishimura
Next to Baker's corpse

Okajima
Maggots of the real DARPA Chef in cell

Onoda
Where Baker is tied up

Sasaki
Picture frames in the Commander's room

Sato
Comm Tower A; roof destroyed by Hind D missiles

Scott Dolph
Deep down in the dark from the walkway between Comm Towers A and B

Shigeno
Heliport,security caera by staircase

Shikama
Electric floor

Shimizu
Wolf dog cave; first crawling point

Shinkawa
Deep in Sniper Wolf's hallway behind the second pillar

Sonoyama
Torture machine

Takade
Ninja room; glass atedga

Tanaka
Heliport; sleeping soldier

Tougo
End of boiler room; in Blast Furnace with the steam

Toyota
Container in the middle of Raven's warehouse

Uehara
Edge of elevator; where the ravens are

Yamashita
Tip of nuke warhead in nuke storage room

Yoshimura
Dead end of air duct

Yoshioka
Bridge on the third floor of the blast furnace

http://www.cheatcc.com/psx/codes/mgs.html & http://metalgear.wikia.com