Slow Motion  -  If you look at the credits, (all the way to the bottom) You'll see the code: Right, Square, Up, Triangle, Right, Square, Up, Triangle

Extra health  -  Buy a soda from the random soda machines. If you are impatient, shoot the soda machine.

Defeating Gonarch  -  The Long Jump module can be used for more than moving between floating platforms. You can get extra height when you jump into a wall or other object. When you first get dumped in Gonarch's lair, to your right you may see white "plants" coming out of the ground. You can use these as stepping stones to get up on the raised area in the middle. This may not easy, but if you get up there Gonarch will have trouble hitting you with any attacks. It is best to use weapons like the crossbow for this part. You can also use the long jump technique to get to other places you are not supposed to, and it is useful for getting past annoying opponents quickly.

Saving ammunition  -  Do not waste precious pistol ammo on headcrabs. Use your crowbar, as it is more useful.

Using guards  -  If you have a guard, get him close enough and he will take care of almost anything for you. The only time you ever need to fire is if the creature is close to the guard.

Man in blue  -  Enable the "Alien mode" code and start a new game. Go through the hall past the main security room and go straight instead of right. A short way down that hall is the man wearing the blue suit. Use your electric attack to break the window and jump in. If you try to attack the man with the blue suit, it will not work.

Dead guard talks  -  After the experiment goes awry, go get the crowbar and come back to the place where the scientist is performing CPR on the dead guard. If you smack the guard with your crowbar, he will talk.

Approach a guard or scientist that tells you information about something. Use a gun that will kill them in one shot, such as the caliber pistol or the crossbow. Shoot them just as you approach them, immediately before they start talking. If done correctly, the guard or scientist will fall to the ground dead, but will still tell you what they originally intended to say.

Look through dead scientist's eyes  -  When you walk around, you will at some point encounter dead scientists, sitting with their back towards a wall. Crouch and then crawl toward the dead scientist. You will glide through him. Continue to walk until you reach the wall, then turn around. You can now see the way the developers have made the skins for eyes and mouth and everything else. You can actually look through the scientist eyes.!

Partial invincibility  -  When in decay mode and are playing both people, the second person is not invincible, even if you enable the "Invincibility" code.

Gameshark Codes

(M) Must Be On EC8783A81431D934 
Unlock Alien Mode Cheat 4CA573BA1456F025 
Unlock Infinite Ammo Cheat 4CA573721456F025 
Unlock Invincibility Cheat 4CA5735A1456F025 
Unlock Invisibility Cheat 4CA5738A1456F025 
Unlock Slow Motion Cheat 4CA573A21456F025 
Unlock Xen Gravity Cheat 4CA573421456F025