In zebrafish,during dark adaptation following bright light adaptation,the dominance of electroretinogram (ERG) b-and dwaves switches. In the early dark adaptation,when visual sensitivity is cone-dominant, both the b-and d-waves are readily recorded. In the late dark adaptation,along with the increase of rod sensitivity,the b-wave becomes dominant whereas the d-wave is gradually lost. The time for the ERG b-and d-wave dominance transition varies between the day and night. The transition requires a longer amount of time in the night and early morning than in the afternoon. This pattern of timing for ERG b-and d-wave dominance transition persists in constant light and can be reversed after exposure to a reversed light-dark cycle. The data suggest that the transition of the dominance of ERG b-and d-waves is regulated by an endogenous circadian clock.