In a recent report, Toxicity Testing in the 21st Century, the National Research Council Committee on Toxicity Testing and Assessment of Environmental Agents (2007) proposed that improved scientific understanding of toxicity pathways was central to the expanded use of predictive, pathway-based bioassays in risk assessment. Toxicity pathways can be viewed as the series of biological changes, spanning across multiple levels of biological organization, that lead from some molecular initiating event (perturbation) to an adverse outcome. A major challenge associated with dose–response modeling and extrapolation from laboratory to real-world conditions has been to understand under what conditions an organism may compensate for, or recover from, a given perturbation and under what conditions the perturbation will lead to an adverse outcome (Andersen et al. 2005). Thus, in developing useful predictive models of toxicity, we need to understand not only the direct effects of a chemical and how they translate into adverse effect, but also the potential mechanisms for compensation and recovery and how they may intersect with other biological pathways and processes. Previous studies with the fathead minnow (Pimephales promelas) have suggested potential compensatory responses to the direct effects of chemicals whose primary mode of action was inhibition of one or more enzymes involved in steroid biosynthesis. For example, the chemical fadrozole (FAD) inhibits aromatase, the enzyme that catalyzes the rate-limiting conversion of testosterone (T) to 17β-estradiol (E2) (Miller 1988). Villeneuve et al. (2006) observed significant, concentration-dependent up-regulation of transcripts coding for the ovarian isoform of aromatase (CYP19A) in female fathead minnows exposed to FAD for 7 days. The increased CYP19A gene expression was associated with an inverted U-shaped concentration–response profile for ovary aromatase activity. Although that study did not examine effects on plasma E2 concentrations, it was noted that the responses would favor increased synthesis of E2 to potentially offset the effect of FAD on aromatase. In another study, Ankley et al. (2007) exposed fathead minnows to the steroidogenesis inhibitor ketoconazole for 21 days. Testosterone production by testis or ovary tissue collected from ketoconazole-exposed fish was significantly reduced compared with control fish. However, there was significant up-regulation of genes coding for cytochrome P450 cholesterol side-chain cleavage (P450scc, CYP11A) and cytochrome P450 c17αhydroxylase, 17,20-lyase, and concentration-dependent proliferation of steroid-producing interstitial cells in the testis of exposed males. As a result, plasma T and E2 concentrations in exposed fish were similar to those of controls, despite the decreased rate of steroid production per unit mass of tissue, suggesting a compensatory response to ketoconazole (Ankley et al. 2007). Both the FAD and ketocona zole studies suggest that fish have the capacity to adapt to and potentially recover from the direct effects of steroidogenesis inhibitors. Aromatase is a key steroidogenic enzyme shown to be subject to inhibition, at least in vitro, by a variety of chemicals present in the environment, including certain pesticides, organochlorines, and organotins (Sanderson 2006). The aim of this study was to develop a more comprehensive understanding of molecular and biochemical responses of fathead minnows to aromatase inhibition, including direct effects, compensation, and/or recovery. We examined a time course of selected gene expression and biochemical responses over the course of an 8-day waterborne exposure to two concentrations of FAD, followed by an 8-day recovery period in control water. The experiment was designed to test several hypotheses: FAD would elicit direct effects consistent with its presumptive mode of action Over time, there would be compensatory molecular responses in females, consistent with an attempt to increase E2 synthesis Compensatory effects at the molecular level would correspond to changes in circulating E2 and/or rates of ex vivo E2 production Effects would be time and concentration dependent There would be recovery after cessation of FAD exposure. Although FAD is a drug with no direct environmental relevance, its specificity makes it a useful model chemical for studying this mode of action. Results of this study provide an improved understanding of the dynamics of biological response to this chemical, and its removal. This knowledge will contribute to formulation of a robust, biologically based toxicity pathway model for the effects of estradiol synthesis inhibitors.