Go into Otacon's lab (where you fought the ninja), then go to the middle desk and look at the monitor. There you will see a Gamecube with a WaveBird controller next to it.

Lay any book on the ground, then go into first person view. Look at the magazine and you will see Alexander Roivas from Eternal Darkness: Sanity's Requiem.

Have either a Eternal Darkness, Legend of Zelda: The Wind Waker, Super Mario Sunshine, or Super Smash Bros. Melee save on the memory card you are using to save, and he will mention them before the boss fight.

In Otacons lab, you can find Mario and Yoshi figurines on top of a computer monitor. If you zoom in on Mario's head and shoot it, a 1-up message will appear over his head.

In the armory room is a room full of nikita equipment. Inside is a locker. Open it up and there will be an Eternal Darkness poster inside.

In the fight against the Ninja you can see a poster hanging from the back wall from Zone of Enders 2: The Second Runner.

In the room where you fight Psycho Mantis, the portraits on the back wall are(left to right) cut-scene director Ryuhei Kitamura, Metal Gear Solid creator Hideo Kojima, and president of Silicon Knights, Denis Dyack.

Right before entering the room where you fight Psycho Mantis, stare at Meryl in first person mode and she will tell you that you are creeping her out and to stop doing that.

In the 'Briefing' section in the title menu, watch every video available, including ones that are added to the list as you've watched more. Once you've seen every video (they will all have a check mark in the box next to them), choose 'Exit' from the list. You will then hear a hidden conversation that explains why Snake's haircut is different from how it appeared in Metal Gear Solid 2.

When you first get to the Holding Cells area, and are going through the ventilation shafts to get to the DARPA Chief's cell, you will see a cutscene of Meryl in a prison cell, doing exercises. After the scene ends, you can look down into the cell to view her exercises anytime. If you go back out the cells the way you came in, and then come back to the vent above Meryl's cell, she will not have any pants on when you look down at her.

If you call Mei Ling three times in a row and choose not to save, she'll yell at you for calling her for no reason. After this, she will stick her tongue out at you occasionally.

Also, right after you follow Meryl into the women's room, but before you trigger the cut scene, call Mei Ling and she'll yell at you for being a pervert, and won't let you save your game.

When fighting the Ninja in the computer room, go to the top right corner of the room and shoot Mario on top of the computer to replenish your health. If you shoot Yoshi, you will hear his voice.

During any cutscene you can zoom the camera in with the R button and move it around with the C-stick.

When you're in the cave area with Meryl, hit her and quickly equip a cardboard box. She'll order a wolf to attack you, but if you're in a box, a wolf will pee on it instead. Now whenever that box is equipped the wolves will no longer attack you.

If you want to easily cross mine fields, but you don't have the mine detector, just crawl on the ground where you believe mines are. Not only will receive no damage, but you'll also automatically collect all mines you crawl over!

When you hang from a ledge, do 100 pull ups (L + R). They have to be good pull ups. Grip Lv "3" is the maximum grip Lv.
Tip: In grip Lv1, you can only fit 4 good pull ups. Dont rush it. Once Snake does a good pull up let go of both buttoms, not just one.

To view Otacon's Ending instead of Meryl's ending after completing the game, submit to Ocelot's torture after being captured.

You can use the cardboard boxes you find for transportation. Simply equip the cardboard box that corresponds with where you want to go (there are three boxes in total - one labeled 'Helipad,' one labeled 'Nuke Building,' and one labeled 'Snowfield') when you are inside the back of one of the parked trucks. Eventually, a guard will appear, and you will be transported to the area labeled on your box.

when you go backtracking through the cavern after getting Sniper wolf's hanky. Equip it and the wolves think you're Sniper wolf and will appear to love you.

If you have a weapon equipped and press "B" to punch Ninja, you'll deal extra damage to him. The nikita is the strongest, though it is very slow. FAMAS and Socom are the same speed, but the FAMAS is more powerful.

While fighting the ninja throw a chaff grenade and he will freeze letting you hit him.Once you hit him he will falls to his knees and won't get up until all the yellow dust from the chaff grenade has fallen.

You can use the thermal goggles to actually see the mines.

Select briefing from the main menu.Then press the LEFT TRIGGER or RIGHT TRIGGER to switch between 4 cameras.Use the analog stick or D-PAD to move the camera around.

If you put on your gas mask while under water, your O2 gauge will decrease slower, just like it would while in a room filled with gas.