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Dicofol, a chlorinated pesticide showing estrogenic activity, was administered orally to virgin pregnant rats at doses of 300, 400, 500, 600, or 700 mg/kg/day for 7 consecutive days to examine its effect on blastocyst implantation. One group of rats received estradiol-17beta (10 microg) for comparison and control animals received similar quantities of olive oil. Autopsy on day 8 revealed that the olive-oil treated rats were pregnant and had a normal number of implantations and a normal duration of diestrus. Treatment with estradiol-17beta completely inhibited implantation, significantly decreased the duration of diestrus with concomitant increase in estrus. Treatment with 300, 400, or 500 mg/kg/d dicofol neither inhibited implantation nor significantly changed diestrus. Treatment with 600 mg dicofol partially inhibited implantation and significantly decreased diestrus with concomitant increase in estrus. Treatment with 700 mg dicofol completely inhibited implantation, and the uterus showed placental scars. This group exhibited a significant decrease in diestrus with concomitant increase in estrus. In all dicofol-treated rats, no significant changes were found in body weight or organ, except for a significant decrease in the weight of the uterus in groups receiving either 700 mg dicofol or estradiol-17beta. The inhibitory effect of dicofol on implantation may be due to an imbalanced estrogen-progesterone ratio, essential for implantation. The pesticide is neither tubal locking nor causes expulsion of the blastocyst from the uterus like estradiol-17beta.