Atrazine, which has been used worldwide as a pesticide, is now known to exert endocrine disrupting (antiandrogenic) effects in mammals. In this study, modifying effects of dietary feeding of 500 and 1000 p.p.m. atrazine on the development of androgen-dependent prostate cancer were investigated using male probasin/SV40 T antigen transgenic (TG) rats. As administration of atrazine has now been identified as causing a decrease in bodyweight, a dietary-restricted TG rat group was also included in order to elucidate the influence of reduction of calorie intake per se on the development of prostate cancer. At week 13, almost the entire lobes of the prostate were occupied with tumor lesions, with no clear intergroup differences in the incidences and multiplicities. Therefore, morphometrical assessment ratios of the prostate epithelial area to the whole prostate tissue area were evaluated. The ratio in the lateral lobe of the 1000 p.p.m. atrazine-treated group was significantly decreased, and there was a tendency to decrease in the ratios in the dorsal lobe of the atrazine-treated groups. However, dietary restriction itself without atrazine treatment caused the same reduction to a similar or greater extent. Testosterone levels were not affected by atrazine administration or dietary restriction. Our results indicate that the observed atrazine-related suppression of prostate carcinogenesis was probably caused by the decrease in calorie intake, rather than by atrazine-related endocrine disruption.