Cheat mode:
     Press Up, Up, Down, Down, B, Down, Up, B at the main menu screen.
A  "Cheat"  option  will  now  appear at the title screen. Select that
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.

Long range stungun:
     Enter FRYINGTIME as a code.

Minigun instead of pistol:
     Enter MINIFUN as a code.

MK3 assault rifle with unlimited grenades:
     Enter ULTIMATEPOWER as a code.

MK4 pistol:
     Enter ZEROTOLERANCE as a code.

Bullets set fire to enemies:
     Enter BURNINGBULLET as a code.

Thermal breather MK2 has better smoke visibility:
     Enter ISEEYOU as a code.

Squeaky voices:
     Enter WHATWASTHAT as a code.

No heads:
     Enter KEEPYOURHEAD as a code.

Disco mode:
     Enter DANCINGFEET as a code.

Terror mode:
     Enter BURNERSREVENGE as a code.