• Big head mode -- Rescue all babies on Level 2: Liberty or Death
• Big gun mode -- Kill all opponents on Level 3: Nuclear Winter
• Flat shade map -- Find all secrets on Level 4: Wetworld
• Ice skin -- Rescue all babies on Level 5: Fallout
• High-speed zombies -- Rescue all babies on Level 8: Dry Town
• Maximum blaster ammo -- Kill all opponents on Level 9: Jailbreak
• Maximum freezer ammo -- Kill all opponents on Level 20: The Brothers Nukem
• Maximum gamma ammo -- Rescue all babies on Level 21: Alien Mothership
• Maximum gattling gun ammo -- Kill all opponents on Level 16: Hydrogen Bomb
• Maximum revolver ammo -- Kill all opponents on Level 12: Probing the Depths
• Maximum rifle ammo -- Rescue all babies on Level 11: Fort Roswell
• Maximum shotgun ammo -- Rescue all babies on Level 10: Up Ship Creek
• Maximum SMG ammo -- Find all secrets on Level 15: Dawn of the Duke
• Maximum sniper ammo -- Rescue all babies on Level 19: Going Down
• Maximum swan-off shotgun ammo -- Kill all opponents on Level 13: White Chapel Killings
• Maximum Volt C. ammo -- Find all secrets on Level 17: The Rack
• Weather -- Find all secrets on Level 6: Under Seige

Bonus Characters
To get extra characters in the multiplayer mode simply beat any level. Then just quit and select multiplayer. You will find a new character available. These characters start with different weapons and different amounts of health.
Submitted by Brian Samuelson

Extreme Mode
Beat every level getting all of the babes and killing all of the enemies and you get extreme mode. It is just a harder level of difficulty.
Submitted by Hightio

FPS Mode and Action Nuk'em Mode
When you beat the game you get these two modes. First Person Mode puts you in the perspective like in Duke Nuk'em 3D. Action Mode makes you have 1-hit kills.

FPS Mode...the easy way
Type these in at the "press start" screen. You will hear a horse neigh if done correctly. Down D-Pad, Up D-Pad, L, B, Z, Left D-Pad, C-up, C-Right, C-left, Z.
Submitted by res

Regain Health
When you shoot a fire hydrant in the game stand next to it and press the A button.This will replenish your health to 100.
Submitted by Jon W Brewbaker

Select Skin Cheat...the easy way
Type this in at the "press start" screen. You will hear a horse neigh if done correctly. Left-C, R, Left D-Pad, Up D-pad, Down D-pad, B, A, Z.
Submitted by res

Titanic Level
To access the Titanic level "GOING DOWN" find all the time machine pieces before you get to "THE RACK" and a second exit portal should appear in the room just off the first.
Submitted by The King

Wetworld Level
To access the "WETWORLD" level play through level 3 'NUCLEAR WINTER' and when you reach the rail tracks leading to the communicator you have to power up drop off onto the roof below where you will see a medi kit. Now jump off this roof into the water (which is only on 1 side) and swim to a floating platform where you will see the level exit.
Submitted by The King