Cheat mode:
Press Left(2), L(2), Right(2), Left(2) at the main menu to access
the cheat menu in the NTSC version of the game.
For the PAL version: press Left, Down, L(2), Up, Right, Left, Up.

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

Invincibility:
Enable the "Cheat mode" code. Press R(7), Left at the main menu
in the NTSC version of the game. A sound will confirm correct code
entry. The invincibility option may now be enabled on the cheat menu.
For the PAL version of the game, press R, C-Right, R, L, R(3), Left.

No monsters:
Enable the "Cheat mode" code. Press L, C-Left, Left, R, C-Right,
Right, Left(2), Right in the NTSC version of the game. The sound
of a siren will confirm correct code entry. Monsters may now be
toggled on the cheat menu. For the PAL version of the game,
press L, C-Up, Left, L, C-Down, R, L, R.

Level select:
Enable the "Cheat mode" code. Press L(3), C-Right, Right, Left(2),
C-Left in the NTSC version of the game. A level select option will
now be available. For the PAL version of the game, press R, L, R,
C-Down, Right, Up, Left, C-Up.

Unlimited jet pack fuel:
Enable the "All items" code while flying with the jet pack.

Huge Duke:
Start a game in co-op or deathmatch mode. Enable the "All Items" code,
as well as the "Friendly Fire" option. Have one player shoot the other
player with the expander weapon. Enable the "Invincibility" code before
the first player explodes, then disable "Invincibility".

Tiny Duke:
Start a game in co-op or deathmatch mode. Enable the "All Items" code,
as well as the "Friendly Fire" option. Have one player shoot the other
player with the shrink weapon. Enable the "Invincibility" code, then
disable "Invincibility" to get a small version of Duke.
Note: Weapons can not be used in this state.

Energy refill:
Destroy any water source, including fountains, fire hydrants, and
toilets. Stand in the water and hold A to slowly restore energy.
Alternately, stand in front of a toilet or urinal and press A.

Fire protection:
Continually jump while over hot spots to avoid damage.

Hint: Entering the L.A.R.D. police station:
Locate the tunnel in the Freeway level. There is a large box on one
side of the tunnel. Jump on top of the box, and from there jump on
to the ledge. On the far side of the ledge there is a pair of night
vision goggles. Jump a little bit above the goggles to enter the
vents of the L.A.R.D. Police station. Use the night vision goggles,
and follow the air ducts and shoot through the fan on the right side
of the wall; or keep going and break the second fan that also on the
right side of the wall. Duke may now explore the interior of the
L.A.R.D. Police Station.