To examine relationships between gonadal stages and the initial appearance and subsequent development of gonadotrophs, hatched larvae of the mummichog Fundulus heteroclitus were reared until first maturity under suitable conditions for maturation (20 degrees C-16L). Evident FSH cells generally appeared 1-2 weeks after hatching (wah), around or slightly before the morphological sex differentiation which occurred at 2 wah. During this period, 3beta-hydroxysteroid dehydrogenase positive cells also appeared in the gonads. While FSH cells existed throughout the early phases of gonadal development such as cortical alveoli formation and basic spermatogenesis, LH cells appeared later (6-12 wah), after the commencement of the early gonadal development. Both FSH cells and LH cells were abundant at 36 wah when the fish had attained full maturity. These results indicate the possibility that FSH is responsible for gonadal differentiation by inducing steroidogenesis in the gonads, implying the importance of FSH on the early phases of gonadal development. These results also suggest cooperation of FSH and LH in later phases of gonadal development such as yolk globule accumulation and active spermatogenesis. The mode of changes in the abundances of the gonadotrophs according to the gonadal development was somewhat different from previously observed changes during the annual reproductive cycle in adult mummichog. Possible complementary roles of the two GTHs in vitellogenesis and spermatogenesis may be involved in the difference by providing flexibility to the controlling mechanism.