In our previous studies, dibutyl phthalate (DBP) was found to be embryolethal and teratogenic in rats. In this study, the effects of DBP on reproductive function were investigated on pregnant and pseudopregnant rats. Rats were given DBP by gastric intubation at 0, 250, 500, 750, 1000, 1250 or 1500 mg/kg on Days 0 to 8 of pregnancy and the pregnancy outcome was determined on Day 20 of pregnancy. The same doses of DBP were given to pseudopregnant rats, with an induced decidual cell response, on Days 0 to 8 of pseudopregnancy, and the uterine weight on Day 9 served as an index of the uterine decidualization. DBP caused significant increases in the incidences of preimplantation loss in females successfully mated at 1250 and 1500 mg/kg and of postimplantation loss in females having implantations at 750 mg/kg and above. The uterine decidualization in pseudopregnant rats was significantly decreased at 750 mg/kg and above. These findings suggest that early embryonic loss due to DBP may be mediated, at least in part, via the suppression of uterine decidualization, an impairment of uterine function.