Battletoads

Six lives:
Hold Down + B + A and press Start at the title screen or continue screen. An explosion will confirm correct code entry. Alternatively, hold Down and press A, B, Start at the title screen or continue screen.

Continue game play:

Allow the continue timer to expire after one of the players loses all lives. Press Start on the next map screen to continue game play.

출처 : http://www.gamewinners.com/nes/Battletoads.htm

출처 : http://oldgamebox.tistory.com/1490