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Cytochrome P450 aromatase catalyzes conversion of C19 androgens to C18 estrogens and is critical for normal reproduction in female vertebrates. Fadrozole is a model aromatase inhibitor that has been shown to suppress estrogen production in the ovaries of fish. However, little is known about the early impacts of aromatase inhibition on steroid production and gene expression in fish. Adult female fathead minnows (Pimephales promelas) were exposed via water to 0, 5, or 50 mu g fadrozole/L for a time-course of 0.5, 1, 2, 4, and 6 h, or 0 or 50 jig fadrozole/L for a time-course of 6, 12, and 24 h. We examined ex vivo ovarian 17 beta-estradiol (E2) and testosterone (T) production, and plasma E2 concentrations from each study. Expression profiles of genes known or hypothesized to be impacted by fadrozole including aromatase (cytochrome P450 [cyp] 19a1a), steriodogenic acute regulatory protein (star), cytochrome P450 side-chain cleavage (cyp11a), cytochrome P450 17 alpha hydroxylase/17,20 lyase (cyp17), and follicle stimulating hormone receptor (fshr) were measured in the ovaries by quantitative real-time polymerase chain reaction (QPCR). In addition, broader ovarian gene expression was examined using a 15k fathead minnow microarray. The 5 mu g/L exposure significantly reduced ex vivo E2 production by 6 h. In the 50 a treatment, ex vivo E2 production was significantly reduced after just 2 h of exposure and remained depressed at all time-points examined through 24 h. Plasma E2 concentrations were significantly reduced as early as 4 h after initiation of exposure to either 5 or 50 jig fadrozole/L and remained depressed throughout 24 h in the 50 mu g/L exposure. Ex vivo T concentrations remained unchanged throughout the time-course. Expression of transcripts involved in steroidogenesis increased within the first 24 h suggesting rapid induction of a mechanism to compensate for fadrozole inhibition of aromatase. Microarray results also showed fadrozole exposure caused concentration- and time-dependent changes in gene expression profiles in many HPG-axis pathways as early as 4 h. This study provides insights into the very rapid effects of aromatase inhibition on steroidogenic processes in fish. Published by Elsevier Inc.