These studies were performed in order to investigate the roles of GnRH and GtH in the regulation of reproductive function in male brown bullhead catfish, Ictalurus nebulosus. Adult catfish were given intravenous injections of salmon GtH (SG-G100) or salmon GnRH (sGnRH), and plasma testosterone, 11-ketotestosterone, and estradiol-17P levels were measured. SG-GlOO elicited parallel increases in testosterone and 11-ketotestosterone, with peak levels between 8-12 hr after treatment. The effects of SG-G100 on plasma testosterone were dose-dependent, whereas increased 11-ketotestosterone was only seen with the two highest doses. Treatment with sGnRH elicited increased testosterone at 4 hr, and 11-ketotestosterone between 4-24 hr posttreatment. No clear dose-response to sGnRH was observed. The effects of SG-GlOO and sGnRH varied with the annual reproductive cycle. During early testicular recrudescence in May, both the pituitary and testis were sensitive to trophic stimulation, with 2to 4-fold increases in plasma androgens following sGnRH and SG-G100. Neither treatment was effective in mature animals trapped in July. In November, despite low basal androgen levels, SG-G100 and sGnRH elicited 2-fold increases in plasma androgen levels. The stimulatory effects of sGnRH, but not SG-G100, were abolished in hypophysectomized fish. These results indicate a functional hypothalamic-pituitary-testicular axis in this species, the components of which vary in sensitivity during the annual reproductive cycle in a manner consistent with the observed cycle of plasma steroids of this species.