This chapter has briefly reviewed the current status of investigations on the hormonal regulation of oocyte growth and maturation in fish (see Figs. 4 and 9). Pituitary gonadotropins are of primary importance in triggering these processes in fish oocytes. In both cases, however, the actions of gonadotropins are not direct, but are mediated by the follicular production of steroidal mediators, estradiol-17 beta (oocyte growth) and 17 alpha,20 beta-DP or 20 beta-S (oocyte maturation). Investigators have established that both estradiol-17 beta and 17 alpha,20 beta-DP are biosynthesized by salmonid ovarian follicles via an interaction of two cell layers, the thecal and granulosa cell layers (two-cell-type model). The granulosa cell layers are the site of production of these two steroidal mediators, but their production depends on the provision of precursor steroids by the thecal cell layers. A distinct steroidogenic shift from estradiol-17 beta to 17 alpha,20 beta-DP, occurring in salmonid ovarian follicles immediately prior to oocyte maturation, is a prerequisite for the growing oocytes to enter the maturation stage, and requires a complex and integrated network of gene regulation involving cell specificity, hormonal regulation, and developmental patterning. The cDNAs for most of the steroidogenic enzymes responsible for estradiol-17 beta and 17 alpha,20 beta-DP biosynthesis have been cloned from rainbow trout ovaries. Our next task is to determine how gonadotropin and other factors act on ovarian follicle cells to turn the expression of these specific genes on and off at specific times during oocyte growth and maturation. Increasing evidence now suggests that a variety of neuromodulatory, autocrine, and paracrine factors may also be involved in the regulation of steroidogenesis in fish ovarian follicles. Molecular biological technologies should be applied to identify these substances. Of considerable interest is the finding that MIH, unlike most steroid hormones, acts on its receptors at the surface of oocytes. Further studies of the association of the MIH-MIH receptor complex with a Gi protein, probably resulting in the inactivation of adenylate cyclase, should lead to a discovery of a new mechanism of steroid hormone action. The early steps following MIH action involve the formation of the major cytoplasmic mediator of MIH, MPF. Fish MPF, like that of Xenopus and starfish, consists of two components: cdc2 kinase and cyclin B. Nevertheless, the mechanism of MIH-induced MPF activation in fish oocytes differs from that in Xenopus and starfish because the appearance of cyclin B protein is a crucial step for 17 alpha,20 beta-DP-induced oocyte maturation in fish.(ABSTRACT TRUNCATED AT 400 WORDS)