At the main menu, press Up, Up, Down, Down, Y, Down, Up, Y to unlock the Cheat option at the title screen. Select the Cheat option, then enter one of the following case-sensitive codes to activate the corresponding cheat function, which can be toggled on or off (except for All levels and emergencies):

All levels and emergencies
Enter KEYTOTHECITY as a code to unlock all levels and emergencies.

Long range stungun
Enter FRYINGTIME as a code.

Minigun instead of pistol
Enter MINIFUN as a code.

Mk. 3 assault rifle with unlimited grenades
Enter ULTIMATEPOWER as a code.

Mk. 4 pistol
Enter ZEROTOLERANCE as a code.

Bullets set fire to enemies
Enter BURNINGBULLET as a code.

Thermal breather Mk. 2 has better smoke visibility
Enter ISEEYOU as a code.

Disco mode
Enter DANCINGFEET as a code.

Terror mode
Enter BURNERSREVENGE as a code.

Squeaky voices
Enter WHATWASTHAT as a code.

No heads
Enter KEEPYOURHEAD as a code.