Progesterone, at five times normal 24 h endogenous production rates, was administered daily to 37 rats over Days 2 to 10, 11 to 17 or 2 to 21 of gestation: term is Day 23. Peanut oil, the vehicle, was administered alone to 18 control rats over Days 2 to 21. The effect of treatment on fetal, placental and corpus luteal weights was examined on Day 22 of gestation. The administration of progesterone had no apparent effect on the maternal weight gain with pregnancy, myometrial, fetal or placental weights. No external malformations or abnormalities of the internal genitalia were detected in any of the fetuses examined. The mean number of dead fetuses per litter, 0.72 ± 0.19 (SEM) in the control rats, was significantly increased to 1.57 ± 0.34 and 2.25 ± 0.54 in rats treated with progesterone over Days 11 to 17 and 2 to 21 respectively. Male fetuses were about 7.4 percent heavier than female fetuses in the control rats but this difference was significantly less in rats treated with progesterone over Days 11 to 17(3.4 percent). Progesterone treatment had no apparent effect on the mean corpus luteal weight per rat. Over all groups combined, the mean corpus luteal weight was negatively related to the number of corpora lutea per rat. This relationship was unlikely to have been due to local nutritional or overcrowding factors since there was no apparent relationship between corpus luteal weight and the number of corpora lutea in the one ovary.