Dr. Mario 64 Codes -

Super hard difficulty:
Select story mode, highlight the "Hard" difficulty setting, then press .

Expert mode:
Select versus mode, highlight a CPU player, then hold and press (for Hard difficulty) or (for Super hard difficulty).

More viruses:
Press or in marathon mode to increase the number of viruses.

Metal Mario:
Successfully complete story mode as Wario under the normal, hard, or super hard difficulty setting without losing or using the redo option. Defeat Metal Mario without losing to unlock him in versus mode.

Vampire Wario:
Successfully complete story mode as Dr. Mario under the normal, hard, or super hard difficulty setting without losing or using the redo option. Defeat Vampire Wario without losing to unlock him Mario in versus mode.