When playing the original NES Metroid, enter the following codes at the password screen to unlock the effects.

• Best ending with no suit: X----- --N?WO dV-Gm9 W01GMI
• Debug Mode: BOMBS_ MISSLE SCREW_ ATACK_
• Hard Mode: 999999 999999 KKKKKK KKKKKK
• Infinite missiles and health with all upgrades: NARPAS SWORD0 000000 000000
• Start with Morph Ball and Bombs: 000000 000000 4G0000 00000H
  
  
• Suitless Samus: JUSTIN BAILEY / ------ ------
• Start with Suitless Samus: 000000 000020 000000 000020
• Start on Norfair with Suitless Samus and infinite missles. No item upgrades except missles: SAMUS8 RIDLEY 444444 444444
• Start at Kraid's Lair with the Long Beam, Ice Beam, Bombs, High Jump Boots, Screw Attack, Varia, Suitless Samus, and 255 Missles: JUSTIN BAILEY ~~~~~~
• Start in Tourian, with Mother Brain defeated: Samus_ Beats_/Mother Brain_ (_=space)

These image galleries are available in the options menu.

• Metroid Fusion Ending Images: Link Metroid: Zero Mission with Metroid Fusion via a Game Boy Advance Game Link cable.
• Metroid: Zero Mission Ending Images: Complete Zero Mission to unlock one ending image. Run through the game eight times to fill the gallery with all eight ending pictures.

Complete one run through Metroid: Zero Mission.

Complete one run through Metroid: Zero Mission.

To open up the various game endings fulfill these requirements.

• Samus in shorts, at a bar: Complete the game in under two hours on Normal with 100%.
• Samus in shorts, in a city: Complete the game in over two hours on Normal or Hard with 100%.
• Samus on a rock: Complete the game on Easy, or complete the game in over four hours in Normal or Hard with more than 15%.
• Samus in a blue jumpsuit: Complete the game in under two hours on Normal or Hard with more than 15%.
• Samus in shorts, looking over a city: Complete the game in under two hours on Hard with 100%.
• Samus in shorts, looking out a window: Complete the game on Normal with less than 15%.
• Samus with a close up on her face without armor: Complete the game on Hard with less than 15%.
• Samus without helmet, looking at a sunset: Complete the game in three hours on Normal or Hard with more than 15%.

Beat Zero Mission once through and begin a new game with the completed save file. View the timer on the map screen.

• Fusion Gallery: Link up with Metroid Fusion
• Gallery: Complete the game once (images will be added when you beat the game differently.
• Hard Mode: Complete the game once on Normal.
• Original Metroid: Complete the game once.
• Sound Test: Complete Hard Mode Once.

If you reach the final boss with 100% of all items collected, the battle will actually be significantly tougher. Not only will the boss take more attacks to kill but it'll also deal a lot more damage, depending on your game difficulty. Unlock the extra Hard mode and reach the boss with 100% completion for a tough battle.

When fighting Ridley, go to the far end of the platform and drop into the lava. Then quickly jump and grab the platform. Ridley can't reach you and you can easily pummel him with Missiles.