The European sea bass is a short-day breeder, a characteristic that is highly valued in aquaculture. A high percentage of males of this species mature precociously before reaching commercial size, resulting in economic losses for fish farmers. We investigated the effects of continuous light (LL) on the circadian variations of several reproductive hormones in males of this species in order to understand how the presumed absence of the melatonin rhythm caused by LL affects their daily profile. The study was conducted during four critical stages of the sea bass reproductive cycle: pre-spermatogenesis (PSpg), spermatogenesis (Spg), spermiation (Spm), and post-spermiation (PSpm). Every 3 h during a complete 24 h cycle, six fish kept under a natural photoperiod (NP), and another six fish kept under LL were anaesthetized, measured, weighed, and bled. The pituitary was removed and frozen at -80 degrees C. The pituitary content of sea bream gonadotrophin-releasing (sbGnRH) and luteinizing hormone (LH), as well as plasma content of LH, testosterone, and 11-ketotestosterone (11-KT) were analyzed by ELISA. The percentage of spermiating males (precocity) per group was determined by periodic abdominal massages of the animals. Our results confirm that LL treatment, maintained from the early stages of development onward, effectively reduces the percentage of precocious male sea bass. As has already been described for caged sea bass, plasma LH showed a clearly marked nocturnal rise near midnight during Spg and Spm during NP, but which was absent under LL. Pituitary sbGnRH and LH content and plasma LH concentration, under both NP and LL, increased during the second half of the reproductive cycle, while sexual steroids were higher at the beginning of the cycle. LL inhibited steroid secretion, especially testosterone secretion, during Spg. In summary, without photoperiod cue, as accomplished by continuous exposure to LL, circadian variations of reproductive hormones appeared altered, causing irregularities in the reproductive process of male sea bass. These findings may have a practical application in aquaculture, namely by applying LL treatment in an effort to reduce the presence of precocious males in a stock.