Several published reports have indicated that certain chloro‐s‐triazine herbicides may alter endocrine function in rats, possibly by androgen receptor binding. In direct tests of estrogenic bioactivity, oral doses of up to 300 mg/kg/d of atrazine, simazine, or the common metabolite diaminochlorotriazine (DACT) did not significantly increase uterine weight of ovariectomized Sprague‐Dawley female rats. The highest dose, which was approximately 10% of the LD50 for these compounds, did cause body weight loss. When administered concomitantly with sc injections of estradiol (2 μg/kg), 300 mg/kg of orally administered chlorotriazines significantly reduced uterine weight in comparison to animals given estrogen alone. Neither atrazine, simazine, nor DACT, at oral doses up to 300 mg/kg/d, stimulated incorporation of [3H]thymidine into uterine DNA of immature Sprague‐Dawley female rats. However, oral treatment at doses of 50 mg/kg and higher significantly reduced thymidine incorporation into uterine DNA extracted from immature rats given a single injection of 0.15 μg estradiol. Oral doses of 300 mg/kg of atrazine, simazine, or DACT significantly reduced expression of progesterone receptor binding in cytosol fractions prepared from uteri of ovariectomized rats injected sc with 1 μg estradiol; 50 mg/kg triazine was not effective in this case. Uterine progesterone receptor levels were not stimulated in rats given oral doses up to 300 mg/kg of these triazines without estradiol injections. These results suggest that atrazine, simazine, and DACT possess no intrinsic estrogenic activity but that they are capable of weak inhibition of estrogen‐stimulated responses in the rat uterus. This inhibition may play a role in the previously observed disruptive actions of chlorotriazines on reproductive endocrine function of female rats.