Goldfish Carassius auratus L., were held for 1 month on one of four different photoperiods. In those groups with diel variations of serum cortisol and thyroxine (approximately 60%) the variations were entrained to the photoperiod; peak titres occurred around light onset, minimum titres at light offset. Fish held on two of the photoperiod regimes had diel variations in serum protein, however, the pattern was not consistent in relation to any aspect of the photoperiod.