Ketoconazole (KTC) is a model pharmaceutical representing imidazole and triazole pesticides, which inhibit fungal growth through blocking a cytochrome P450 (CYP)-mediated step in ergosterol biosynthesis. Several of these fungicides have been shown to be reversible inhibitors of CYPs in vertebrates (primarily mammals), including CYP isoforms involved in the pathway that converts cholesterol to active sex steroids. In these studies, we assessed the effects of KTC on aspects of steroidogenesis and reproductive function in the fathead minnow (Pimephales promelas). Exposure of spawning adults to the fungicide for 21 d significantly decreased egg production at a water concentration as low as 25 μg/L. Despite evidence of reduced ex vivo testosterone production by gonads from KTC-exposed fathead minnows, circulating plasma concentrations of sex steroids (testosterone, 17β-estradiol) were not affected. Exposure to KTC caused an increase in the gonadosomatic index in both sexes and, in males, the fungicide caused a marked proliferation of interstitial (Leydig) cells. In addition, mRNA transcripts for two key steroidogenic enzymes, cytochrome P450 side-chain cleavage (CYP11A) and cytochrome P450 c17α hydroxylase/17,20 lyase (CYP17), were elevated by exposure to KTC. Both the changes in transcript levels and proliferation of gonad tissue represent potential adaptive or compensatory responses to impaired steroidogenic capacity. Overall our data indicate that, although KTC does adversely affect steroidogenesis and reproduction in the fathead minnow, the fish can compensate to some degree to mitigate effects of the fungicide. This has important implications for the interpretation of data from tests with endocrine-active chemicals.