All Items
Press R1(5), L2(5).
All Keys
Press Up, Right, Up, Left, Down, Up, Right, Left, Right, Down.
All Weapons
Press L1, L2, Up, L1, L2, Down, R1, Right, R2, Left.
Beat the Game
While playing hold L2 + R2 then press Square, Triangle, Circle, Circle, Start.
Big Head Duke
Press R1(9), Up
Big Head Opponents
Press R1(9), Left
Double Damage
Press L2, R2, L2, R2, L2, R2, L2, R2, L2, R2.
Infinite Ammo
Press [Left, Right, Left, Right, Select] twice.
Invincibility
Press L2, R1, L1, R2, Up, Down, Up, Down, Select(2).
Invisibility
Press L1, R1, L1, R1, L1, R1, L1, R1, L1, R1.
Invulnerability
Enter one of the following codes while the game is paused:
First pause the game and press: L2, R1, L1, R2, UP, DOWN, UP, DOWN, SELECT, SELECT. And it should say Invulnerability.
Level Select
Press Down(9), Up {See Note Below} Note: After entering this code, exit the game. Then at the bottom of the main menu there will be a new option called "Time To Kill". Select this new option, then press Left or Right to choose a level and press X to go to the level.
Select Powered-Up Weapons
Press Right(2), Left, Right(2), Left, Right(2), Left.
Small Head Duke
Press R1(9), Down.
Small Head Opponents
Press R1(9), Right
Temporary Invincibility
Press R1, L2, L1, L2, R1, L1, R1, L2, L1, L2.
View Scenes
Intro Scene: Pause game, press select, up nine times, select, L1.
Victory Scene: Pause game, Select, up nine times, select, L2.
View Credits: Pause game, Select, up nine times, select, R1.
Make Duke Wear A Kilt:Pause game, Select, up nine times, select, R2