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In a recent survey of pesticide concentrations in aquatic ecosystemsof Central America, the antifungal triazole compound propiconazole was found to be the most widely distributed pesticide. Previously, technical grade propiconazole (TGP) has been shown to modulate cytochrome P450 activity in mammals and birds. The present study investigated the concentration- and time-dependent effects of TGP on hepatic cytochrome CYP1A gene expression and catalytic activity in rainbow trout (Oncorhyncus mykiss). The TGP produced both a mixed-pattern response and a biphasic response for CYP1A expression following waterborne exposure. Evidence for inhibitory complex formation with cytochrome P450 may explain the occurrence of the mixed-pattern response. To further characterize the influence of TGP on CYP1A expression, a comparison was made between TGP and analytical grade propiconazole (AGP) with in vivo and in vitro assays. This comparison demonstrated that inductionof the CYP1A gene resulted from unidentified compounds in TGP. SinceTGP can be coapplied with other agricultural pesticides, either inhibition or induction of CYP1A protein activity by TGP and potentially other cytochrome P450 isoenzymes may lead to unexpected toxicologicalinteractions. [ABSTRACT FROM AUTHOR]