Pause the game and enter a code to activate that code.

Level Select:
Down(x9), Up
Then exit the game and at the bottom of the Main Menu select the new option "Time to Kill. You can then toggle between levels by pressing right and left. NOTE: You only have to do this addtional step for the level select.

Powered Up Weapons:
Right(x2), Left, Right(x2), Left, Right(x2), Left

Total Invincibility:
L2, R1, L1, R2, Up, Down, Up, Down, Select(x2)

Complete Invisibility:
L1, R1, L1, R1, L1, R1, L1, R1, L1, R1

All Weapons:
L1, L2, Up, L1, L2, Down, R1, Right, R2, Left

All Items:
R1(x5), L2(x5)

All Keys:
Up, Right, Up, Left, Down, Up, Right, Left, Right, Down

Unlimited Ammo:
Left, Right, Left, Right, Select, Left, Right, Left Right, Select

Small Head Duke:
R1(x9), Down

Big Head Duke:
R1(x9), Up Damage X2:
L2, R2, L2, R2, L2, R2, L2, R2, L2, R2