Sound test:
Press II, I, II, I, II, I, Run at the title screen.

Unlimited credits:
Press Up, Up, Down, Right, Right, I, Up, Left, II, Run at the title screen.

Full life:
Press Down, Left, Up, Right, Run at the title screen.

Nine credits:
Press Up, Down, Left, Right, Run at the title screen.

Full weapon:
Press I, I, I, II, Right, II, Select, Run at the title screen.

Easy difficulty level:
Press Select, II, I, Down, Select, Run at the title screen.

Hard difficulty level:
Press Select, I, II, Up, Select, Run at the title screen.

No power suit:
Enter I, II, I, II, I, II, I, II, I, II, I, II, I, II, I, II, Up, Left, Down, Right, Run at the title screen.