Enter these codes at the main title screen (when the words "Press Start" appear). Entering the same code again will disable it.

Jungle Stadium- up, down,left,right,X,Y
Pacific Atoll Stadium- up,up,left,left,Y,Y
Oasis Stadium- up,up,down,down,X,X
Alpen Castle Stadium- up,up,up,down,X,X
Riviera Ruins Stadium- Up,down,down,right,Y,X
Reactor Core Stadium- up,left,left,right,X,Y


unlock ALL stadiums-up,up,up,up,X,X