To unlock the stadiums listed below, enter the button code at the main title screen:

All: R, R, RIGHT, RIGHT, UP, UP, UP, UP, UP, X, X
Alpen Castle: UP, UP, UP, DOWN, X, X
Jungle Canopy: L, R, UP, DOWN, LEFT, RIGHT, X, Y
Pacific Atoll: UP, UP, LEFT, LEFT, Y, Y,
Reactor Core: UP, LEFT, LEFT, RIGHT, X, Y
Riviera Ruins: UP, DOWN, DOWN, RIGHT, Y, X
Tribal Oasis: L, R, UP, UP, DOWN, DOWN, X, X