Pause the game (press the F1 key) and enter one of the following passwords and then unpause the game to make them work.

After defeating the Metal Gear, use the cigarettes and you'll get an extra 2000 units of time.

To get infinite expendable items, first pick up the item, then press F1 to pause the game, then F4 to enter the Load screen then press ESC to leave it and press F1 to unpause. If done right, the item will appear once again for you to pick up!

To pass through electric fields without taking any damage, press F1 to pause the game, then F4 to enter the Load screen then press ESC to leave it and press F1 to unpause.

Enter the following at the password screen...

In the areas where the black holes quickly open up to swallow you just simply pause the game and unpause. When you return to game play the holes will be frozen in the state of expansion they were in when you paused. They will be invisible but smaller. Simply side step around where you saw them last and you will be in the clear.

When fighting the Bull-Tank you can reset its position by accessing your Sub-menu. This comes in handy if it gets too close, due to it having a 1-touch kill on you. To defeat it, use your Grenade Launcher.

When entering the Super Computer's room, stay in the door frame and push right on the control pad. You'll automatically warp to the final boss and won't have to worry about the base's self-destruc countdown that the computer's destruction usually starts. Now you have all the time you need to beat the last boss.

When you escape the Final Level have your character smoke a cigarette and the time will be reset!