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Some investigators have suggested that the triazine herbicide atrazine can cause demasculinization of male amphibians via upregulation of the enzyme aromatase. Male adult African clawed frogs (Xenopus laevis) were exposed to three nominal concentrations of atrazine (1, 25, or 250μg atrazine/l) for 36 days, and testicular aromatase activity and CYP19 gene expression, as well as concentrations of the plasma sex steroids testosterone (T) and 17β-estradiol (E2), and gonad size (GSI) were measured. There were no effects on any of the parameters measured, with the exception of plasma T concentrations. Plasma T concentrations in X. laevis exposed to the greatest concentration of atrazine were significantly less (p = 0.034) than those in untreated frogs. Both CYP19 gene expression and aromatase activities were low regardless of treatment, and neither parameter correlated with the other. We conclude that …