Under constant short photoperiod, the spawning time of 2‐year‐old sea bass Dicentrarchus labrax was advanced as compared to controls, whereas spawnings were delayed under constant long photoperiod. High plasma levels of 17β‐oestradiol (E2/) and testosterone (T) in females were coincident with the appearance of vitellogenic oocytes in the ovary, while high levels of 11‐ketotestosterone (11‐KT) and T in males were coincident with the presence of spermiating males. Although plasma levels of E2 in females and 11‐KT in males were low during the remainder of the cycle, levels of T were always >1 ng ml−1 in both sexes, suggesting that T could play an important role during the initial stages of gonadal development. The profiles of E2 and T in females and 11‐KT and T in males exposed to constant short days were similar to those in the control group, but fish which were maintained under constant long photoperiods showed a bimodal pattern of these steroids. The results obtained from fish exposed to constant photoperiod regimes provide further evidence that an endogenous process could be operating to control the reproduction of sea bass.