In the original 1980s Metroid game, the Zero Suit wasn't the same as modern depictions. It looked more like a one-piece bathing suit than the modern skin-tight jumpsuit. To get Samus to wear it, use any of the following passwords:
Samus will spawn suitless (pink leotard, brown hair) with in Norfair with 4 (empty) Energy Tanks, 255 Missiles, the Wave Beam, Long Beam, Morph Ball Bombs, Jump Boots, and Screw Attack.
Samus will spawn suitless in Brinstar with about 200 Missiles, 3 available Energy Tanks, and all upgrades excluding the Long Beam, Morph Ball Bombs, Screw Attack, and Varia Suit. With this code, it is not necessary that Samus will go and defeat Kraid or Ridley; the gateway to Tourian is open.
Samus will spawn suitless in Kraid's Lair with 36 Missiles, 1 Energy Tank, the Wave Beam with no Long Beam, the Screw Attack with no High Jump Boots, and Bombs with no Morph Ball. This will also increase the difficulty level and change the ending.
Samus will spawn suitless in Brinstar with no upgrades, Energy Tanks, or Missiles.