My study demonstrated that three selected PAHs (β-naphthoflavone [βNF], naphthalene, retene) enhance testosterone (T) production in sexually mature male goldfish, Carassius auratus , and rainbow trout, Oncorhynchus mykiss . In vitro incubations of goldfish testis tissue exposed to either PAH resulted in biphasic potentiation of gonadotropin (GtH)-stimulated T production, with no effect on basal levels. Experiments with rainbow trout showed a decreased sensitivity responding only to the highest concentrations tested. In vivo exposure of goldfish to βNF also resulted in biphasic increases in plasma T levels. Studies of possible sites of action at the level of the testis showed that βNF exerted its most potent action when T production was initiated by a ligand such as GtH. Within the steroidogenic pathway, βNF potentiation of GtH-stimulated T production may be mediated partially by increases in cAMP production and enzyme activity and/or synthesis, but appear to be independent of calcium and the aryl hydrocarbon receptor (AhR). My investigation established PAHs as reproductive endocrine disruptors in sexually mature male fish and provided preliminary results into possible sites of action.