There is a growing international concern that commonly used environmental contaminants have the potential to disrupt the development and functioning of the reproductive system in amphibians. One such chemical of interests is the herbicide atrazine. Effects of atrazine on sex differentiation were studied using wild-type Xenopus laevis tadpoles and all-ZZ male cohorts of X. laevis tadpoles, produced by mating wild-type ZZ male to sex-reversed ZZ male (female phenotype). Stage 49 tadpoles were exposed to 0.1-100 ppb atrazine or 0.27 ppb (1 nM) 17beta-estradiol (E(2)) until all larvae completed metamorphosis (stage 66). Metamorphosis, gonadal morphology and histology, CYP19 (P450 aromatase) mRNA induction, and hepatic vitellogenin (VTG) induction were investigated. Effects of atrazine on VTG-induction were also assessed in vitro in primary-cultured X. laevis hepatocytes. Atrazine had no effect on metamorphosis of developing wild-type or all-male X. laevis larvae. Statistical increase in female ratios was observed in 10 and 100 ppb atrazine groups in comparison with control group. While no hermaphroditic froglet was observed in all atrazine groups. In ZZ males, sex reversal was induced by 0.27 ppb E(2), but not by atrazine at concentrations of 0.1 and 1.0 ppb. In addition, neither P450 aromatase mRNA in the gonad nor hepatic VTG were induced by atrazine. Furthermore, VTG was not induced by 1000 ppb atrazine in primary-cultured hepatocytes. Our results indicate that female ratios in developing X. laevis tadpoles were increased by 10 and 100 ppb atrazine under the present experimental conditions. While the other endpoints showed no effect in the range of 0.1-100 ppb atrazine. These results suggest that effect of atrazine on sexual differentiation was not caused by estrogenic action and has no induction ability of P450 aromatase gene in gonad.