Mayhem is an arcade-style platform shooter. The world is being invaded by killer bunnies, and you have to stop them! Armed with 8 deadly weapons you must try to survive as the bunnies invade. The goal of the game is simply to survive for as long as possible, kill as many bunnies as you can and get a high score. Achieving certain milestone scores unlocks bonuses such as cheat codes and artwork.
Bunnies keep appearing out of portals infinitely, and if you don't kill them quickly you will soon be overrun. There are also smaller portals which every now and then drop ammo for your guns. There are also warp portals that teleport you or the bunnies across the stage when touched.
Health cannot be replenished, but you can find shields which will protect you against one additional hit. There is also money strewn across the stage, which you can pick up for some extra points.
There is a selection of two stages - the caves and the forest - with a secret third level being unlockable.