Invincibility
To get the invincibility code players must first enter the cheat menu code ( Press the following at the main menu to access the hidden cheat menu: LEFT, LEFT, L, L, RIGHT, RIGHT, LEFT, LEFT. Now the cheat menu will be accessible). Then simply hit R button seven times in a row and then press left on the directional pad. A sound should signify that it worked. Now go into the cheat menu and turn invincibility on.

Switch monsters off
Go to the cheat menu (to get the cheat menu it self do this code on the main menu: LEFT, LEFT, L, L, RIGHT, RIGHT, LEFT, LEFT. Now you can get in the cheat menu.) Then hit these buttons in this order: L, LEFT C, LEFT, R, RIGHT C, RIGHT, LEFT, LEFT, RIGHT. You will here a siren if you did the code correctly. Then go in the cheat menu and now you can turn the monsters on and off.

Size Glitch
There is a way to play big or small in Duke Nukem. This code will work on co-op or deathmatch. To do this code in co-op you need Invincibility and items codes. Turn on friendly fire, then use the weapons code, shoot your buddy with the shrinker, or expander. Small Duke: Shoot, put on invincibility then turn it off again. The player will not grow back (the drawback: you can't have any weapons). Big Duke: Get the expander, (with God mode off) and shoot the other Duke until he starts to expand. Just before he explodes put on God mode, stop shooting, then turn off god mode. You now have a giant duke that still can be killed. (Note, you may have to try it a few times to get it right).

Duke Nukem 64 Cheat Menu
Press the following at the main menu to access the hidden cheat menu: LEFT, LEFT, L, L, RIGHT, RIGHT, LEFT, LEFT. Now the cheat menu will be accessible.

All Items
Enable the " cheat mode " code. Press R,
C-right, Right, L, C-left, left, C-right. At the cheat menu.

Level Select
To pick any level on Duke Nukem 64 enter this at the title screen: L,L,L,Right C,Right,Left,left,Left C

Unlimited Ammo
At the main menu press A(5),L,C(UP),R,C(DOWN),B(4)
Warning: Reported as not working.

Turn Items On
Perform the code to access the cheat menu (from main menu: LEFT, LEFT, L, L, RIGHT, RIGHT, LEFT, LEFT). Now at the main menu input: R, Right C, RIGHT, L, Left C, LEFT, Right C, RIGHT. This will enable all items to be toggled on or off.

Unlimited Lives (reportedly only works in UK Version)
At the title screen press (R) 7 times then press left on the directional pad

Giant Duke, Tiny Duke, Dark Duke, Dying Duke, etc.
First enter the cheat for invisibility, all weapons, etc. Then start a multiplayer Dukematch. Go to options and turn invisibility off only if it isn't already off. Now fire an expander shot at your opponent, just as he is about to explode pause the game and turn invisibility back on. You will now have a Giant Duke. Pause the game again and turn invisibility off and you will notice that the opponent won't explode. Repeat this action with the shrinker, the nuke and the rocket(Right when he explodes put invisibility on- you will have Duke that looks like he is constantly getting shot and bleeding!)

The Dukes of Nukem and their Weapons Duplication
In Duke Nukem, there are 4 "secret characters" called the Dukes of Nukem. They aren't really real, but they require you to be able to shoot and kill your friend. And be able to make him invincible. How to access each:

Duke of Enormity:
This is how to make the Duke of Enormity, the really big one. Turn off Friendly Fire (a.k.a. FF) and Invincibility (a.k.a. 8). Then fire an expander at your friend. Before he explodes, but while he's big, turn on 8. Unpause it, then pause it again, and turn 8 off. Now he's really big.

Duke of Shrinkery:
This is how to access the Duke of Shrinkery, the unarmed small one. Have FF and 8 off, then shoot your buddy with the Shrinker. While he's small, turn 8 on. Unpause, then turn 8 off again. He's small, but he don't have no puny weaponry.

Duke of Mortality:
This is how to be the Duke of Mortality, the one who is always dying. FF and 8 off. Kill your buddy and tell him not to press start. Turn on 8, unpause, turn it off again, and you have your constantly bleeding Duke.

Duke of Darkness:
This is how to be the Duke of Darkness, the completely black espionage one. This is tricky. Take out the Plasma Cannon (FF8OFF).
Charge it up fully, and fire! When your buddy's screen is yellow, turn on 8, unpause, then turn it on again. VIOLA! The Shadow Duke!

When you create a Duke of Mortality, he leaves behind 2 clones of his weapon. Tell the guy with the most weaponry to be the Duke of Mortality, because he's the munitions guy.