Methoxychlor (MXC) has been shown to have adverse effects on reproductive functions. However, it has not been fully determined whether the effects of MXC on reproduction are due to its estrogenic or antiestrogenic effects. Therefore, to further elucidate the estrogenic action of this pesticide in the mouse, we studied the effect of MXC on implantation and embryo development. MXC was found to initiate implantation in most delayed implanting mice at 400 microg/g body weight. However, at the higher dose of 800 microg/g body weight, MXC initiated implantation in only 50% of animals and the number of embryos implanting was significantly decreased (P < 0.05). It was determined that MXC inhibited implantation in intact pregnant mice only when given on Day 1 or Day 2 at 800 microg/g but not at lower doses or later in the preimplantation period. Embryonic development and transport were delayed on Days 3 and 4 in these animals. Finally, reciprocal embryo transfers with donor embryos recovered from MXC-treated animals (800 microg/g body weight on Day 1) transferred to untreated recipients resulted in no implantation compared to 79% implantation when donor embryos were treated with vehicle. These data indicate that MXC acts as an estrogen agonist at the level of the uterus and oviduct but as an antiestrogen in the ovary. In addition, MXC appears to alter normal preimplantation embryonic development. These results suggest the need for further studies to assess the mechanism of action of MXC in preimplantation embryos.