The aim of the present study was to investigate the ability of the chloroacetanilide herbicide acetochlor to interact with the endocrine system. The modulation of the binding of [3H]estradiol-17beta to protamine sulphate-precipitated uterine nuclear and cytoplasmic estrogen receptors was analysed for this purpose. Two different stages of reproductive life cycle of female rats, virgin and uniparous, were used. Our results demonstrate that acetochlor interacts in a specific manner with high-affinity limited-capacity uterine estrogen receptors. A significant difference (p < 0.001) in estrogen receptor density was observed between two control groups: uniparous rats (Bmax = 43.634 +/- 9.516) and virgin rats (Bmax = 154.375 +/- 21.462), suggesting an intrinsic difference in the nuclear estrogen receptor levels between female rats in different reproductive life cycle stages. Consequently, a different response to acetochlor treatment was noted. Exposure to acetochlor significantly (p < 0.001) increased estrogen receptor binding activity in the nuclear fraction of uniparous female rats (Bmax = 123.324 +/- 5.666) in comparison to control (Bmax = 43.634 +/- 9.516). In exposed virgin female rats, no significant difference was detected when compared to the corresponding control group. These results should prompt us to more thoroughly evaluate potential hazards of acetochlor to human and wildlife endocrine systems.