Ratchet Monkey
Hold the 4 shoulder buttons at the start screen (New, Continue, Options, etc.) and press start and a password screen will appear. Enter RATCH in the field. This will make a Ratch Monkey appear in the last levels of the hidden room.

Mesaru Gear
You can buy this after catching Specter. It cost 573 credits.

Survival Mode
You have to make sure you capture all apes and catch Specter a second time.

Croc Cap and Infinite Items
You must rescue all 20 hostages in Mesal Gear Solid.

Hard Mode in Mesal Gear Solid
You must complete Mesal Gear Solid on Normal Mode once.

Super Monkey Throw
You can purchase this in the Hobby Shop for 1500 credits.

Utlim-ape Fighting
You can buy this in Hobby Shop for 1500 credits.

Secret Passwords (North American version)
On the Title Screen, press L1, R1, L2, R2, SELECT, and START at the same time to bring up the Secret Password Screen. Here are some secret passwords (case-sensitive, and watch the spaces):

"krops" - Make Spork appear in the Hide-n-Seek Forest stage.

"redman" - Make Pipotron Red appear in the Toytown stage. Alternate "redmon".

"coolblue" - Make Pipotron Blue appear in the Air Squadron stage.

"yellowy" - Make Pipotron Yellow appear in the Winterville stage.

"SAL3000" - Make SAL-3000 appear in the Space TV Fortress stage.

"2 snakes" - Unlock the Mesal Gear Solid cutscenes. They are Movie #28 (Home Theater) and the last two cinema movies (Simian Cinema).

"grobyc" - Make SAL-1000 appear in the Castle stage.

"2nd man" - Make Shimmy appear in the Winterville stage.

"blackout" - Make Dark Master appear in the Kung Fu Alley stage.

Alternate Backgrounds
To unlock different start-up backgrounds with different scenes being shown, go to the game’s main menu (where is asks you to Press Start), but don’t press anything. When it goes to an in-game demo, press start, and the background of the main menu should be different.