The effects of recombinant salmon growth hormone (sGH) on plasma sex steroid levels and gonadal function were investigated in hypophysectomized Fundulus heteroclitus. Effects of sGH were compared to those of purified chum salmon prolactin (sPRL), Atlantic salmon gonadotropin (sGTH), and salmon pituitary extract (sPE). Treatment with sGH significantly increased plasma concentrations of testosterone in males and estradiol-17 beta in females; sPRL had similar effects on testosterone levels in males. Further, treatment with these hormones prevented the decline in gonadal weight observed after hypophysectomy in both males and females. In vivo treatment of male fish with sGH also augmented testosterone and 11-ketotestosterone production by testis tissue subsequently incubated in vitro. Direct action(s) on gonadal steroidogenesis was examined by incubating gonadal tissues from hypophysectomized fish in vitro with various hormones. sGH significantly stimulated the in vitro production of testosterone and 11-ketotestosterone by testis, and estradiol-17 beta by ovary. sPE and sGTH also stimulated gonadal steroidogenesis, whereas sPRL and bovine GH had no significant effect. By comparison, rainbow trout gonads also produced increased amounts of steroids when treated with sGH in vitro. The use of a cloned GH rules out contamination by other pituitary hormones. These results, therefore, demonstrate that recombinant salmon growth hormone possesses steroidogenic and gonadotropic activity. Purified sPRL also has steroidogenic and gonadotropic actions. However, the significance of these effects of teleost GH and PRL is not known.