At the main menu enter UP,UP, DOWN, DOWN, CIRCLE, DOWN, UP, CIRCLE. Now, select the CHEAT section from the main menu and enter the following passwords for the corresponding modifications.

DANCINGFEET: activate disco mode
BURNERSREVENGE: activate terror mode
BURNINGBULLET: bullets set fire to enemies, like one hit kills
KEEPYOURHEAD: no heads
ISEEYOU: thermal breather MK2 bettter visibilty in smoke
WHATWASTHAT: turns on squeaky voices
FRYINGTIME: unlock long range stungun
MINIFUN: unlock minigun, replaces the pistol
ULTIMATEPOWER: unlock MK3 assault rifle with unlimited grenades
ZEROTOLERANCE: unlock MK4 pistol
KEYTOTHECITY: unlocks all levels and emergencies