Cheat menu
At the main menu, press Left(2), L(2), Right(2), Left(2) in the NTSC version of the game. For the PAL version of the game, press Left, Down, L(2), Up, Right, Left, Up.

All items
Enable the "Cheat menu" code. Then at the main menu, press R, C-Right , Right, L, C-Left , Left, C-Right, Right in the NTSC version of the game. For the PAL version of the game, press R, C-Right, Down, L, C-Up, Left, C-Right, Left.

Invincibility
Enable the "Cheat menu" code. Then at the main menu, press R(7), Left in the NTSC version of the game. If you entered the code correctly, you will hear a sound. Now an option for invincibility will appear on the cheat menu. For the PAL version of the game, press R, C-Right, R, L, R(3), Left.

Level select
Enable the "Cheat menu" code. Then, press L(3), C-Right, Right, Left(2), C-Left in the NTSC version of the game. For the PAL version of the game, press R, L, R, C-Down, Right, Up, Left, C-Up.

No monsters
Enable the "Cheat menu" code. Then, press L, C-Left, Left, R, C-Right, Right, Left(2), Right in the NTSC version of the game. If you entered the code correctly, you will hear the sound of a siren. Now an option to toggle monsters on/off will appear on the cheat menu. For the PAL version of the game, press L, C-Up, Left, L, C-Down, R, L, R.

Infinite jet pack fuel
While flying with the jet pack, enable the "All items" code.

Big Duke
Start a game in co-op or deathmatch mode. Enable the "All items" code and the "Friendly Fire" option. Now have one player shoot the other player with the expander gun. Enable the "Invincibility" code before the first player explodes, then disable "Invincibility".

Small Duke
Start a game in co-op or deathmatch mode. Enable the "All items" code and the "Friendly Fire" option. Now have one player shoot the other player with the shrinker gun. Enable the "Invincibility" code, then disable "Invincibility".

Refill health
Destroy a fire hydrant or another water source. Then, stand in the water and hold A to slowly restore health. Alternatively, stand in front of a toilet or urinal and press A.