The pesticide methoxychlor (MXC) is a proestrogen which is metabolized to a compound that has been shown to exhibit estrogenic activity in vivo and in vitro. Following long-term exposure of female rats to MXC, fertility is reduced and fetotoxicity is evident. However, the effects of MXC on several aspects of maternal reproductive physiology including the decidual cell response (DCR), ovarian weight, serum progester-one levels, and corpora lutea maintenance, have not been previously described. In the present study, the ability of MXC to interfere with progestational events essential for implantation and the maintenance of pregnancy in the rat was investigated. MXC was administered to pseudopregnant rats, decidualization was induced on day 4, and the DCR and related parameters were evaluated following sacrifice on day 9. DCR induction during estrone adminstration served as a positive control.