Gonadotropins, FSH and LH, play a central role in mammalian reproductive process. In teleost, as in mammals, two distinct gonadotropins, GTH-I and GTH-II, have been isolated and characterized in many teleost species. However, with the exception of salmonids, the physiological significance of GTH-I and GTH-II in reproductive process is unclear at this time. This article focuses on physiological roles of GTH-I and GTH-II in male and female red seabream. The expression profiles of GTH subunit genes in red seabream during sexual maturation revealed that the levels of Iβ mRNA of male increased in association with gonadal development, whereas those of female were maintained at low levels. In contrast, IIβ mRNA levels of both sexes were maintained at high levels from the beginning of gametogenesis to spawning season. Furthermore, in vitro studies demonstrated that GTH-II is more potent than GTH-I in stimulating estradiol-17β production and cytochrome P 450 aromatase expression by vitellogenic follicles, while both GTH-I and GTH-II have similar potency in 11-ketotestosterone production in testicular slices. In addition, GTH-II, but not GTH-I, induced in vitro final oocyte maturation. These results suggest that GTH-II have important roles in reproduction of both male and female red seabream, while GTH-I shows sexual dimorphism in the expression of gonadotropin subunit genes and physiological function; GTH-I is functional molecule in male, but not in female red seabream.