Three different estrogen receptors (ERs) have been cloned and characterized in teleosts fish, i.e. ERalpha, ERbeta or ERbeta1 and ERgamma or ERbeta2. In order to study the sea bass ER subtype involved in the regulation of gonadotropin production, as well as to elucidate the possible involved neuronal pathways, we characterized the transactivation properties of the cloned sea bass ERalpha (sbERalpha) and studied its distribution in the brain and gonadotropic cells of the sea bass by in situ hybridization. The results revealed that sbERalpha transactivates promoters containing estradiol responsive elements (ERE) in a dose-response manner. The sbERalpha showed the highest affinity for 17-beta-estradiol. In situ hybridization studies demonstrated that ERalpha mRNA positive neurons are widely distributed within the sea bass brain, including the telencephalon, preoptic area, thalamus, hypothalamus, mesencephalic tectum and tegmentum and rhombencephalon. New estrogen dependent nuclei were described in all above areas. The sbERalpha was profusely expressed in the main neuroendocrine areas such as the preoptic area and hypothalamus, thus suggesting the steroidal modulation of the hypophysiotropic neurons. The presence of sbERalpha expression in the FSHbeta and LHbeta cells suggests a direct effect of estrogens in the control of gonadotropin hormone synthesis.