Three molecular variants of GnRH in the brain (sbGnRH, sGnRH, and cGnRH-II) and two forms in the pituitary (sbGnRH and sGnRH) were detected in protandrous black porgy, Acanthopagrus schlegeli using chromatographic and immunological methods. In juvenile fish, brain sbGnRH, sGnRH, and cGnRH-II levels increased in May and reached their highest levels in July and August (the nonspawning season) and in January through March (the spawning season). In fish aged 1 yr and older, high levels of brain sbGnRH and sGnRH were detected in September, November, and February–March, but the levels of brain cGnRH-II remained constant. A gradual increase in pituitary sbGnRH was detected in juvenile fish from July to March. In fish aged 1 yr, pituitary sbGnRH levels were high in September and March–May, but low in January–February. A close correlation between pituitary sbGnRH and plasma LH levels was found in juvenile fish and in those aged 1 yr. In fish aged 2 yr, significantly lower levels of plasma LH was detected during the nonspawning period in fish that changed sex compared with the fish that remained as males. Higher plasma LH levels were detected in the sex-changing fish from artificially sex-reversed female to male. FSH receptor and LH receptor transcripts were higher in bisexual testicular tissue than in ovarian tissue in 2 -yr-old fish. Direct effects of hCG on sex change were studied and the results show that exogenous hCG did not stimulate gonadal aromatase activity in 2 -yr-old fish. Therefore, it is suggested that high and basal levels of plasma LH during the nonspawning season correlate with the development of male and female gonad, respectively, in black porgy. This important role of the neuroendocrine system in sex change (for male direction) is proposed in hermaphroditic fish.