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Experiment 1, in which exogenous estradiol was administered, failed to demonstrate an increase in placental weights similar to that obtained previously using Meishan gilts. Estrogen levels in the uterus of the Meishan from about Day 11 to Day 13 of gestation are significantly lower than those in the uterus of white crossbred gilts. The amount of estrogen already present in white crossbred gilts on Days 11 and 12 may be sufficient to saturate estrogen receptors, thus explaining the lack of effect of exogenous estrogen. An alternative test of the effect of estrogen during this period might be to determine the effect of an estrogen antagonist on placental weights and uterine capacity during later pregnancy. Unfortunately, many of the known estrogen antagonists (e.g., tamoxifen) have some estrogen agonist activity in swine. However, the effects of these and other estrogen antagonists on placental weight in midpregnancy have never been evaluated. Results of the first and fourth experiments indicated that both progesterone and mifepristone treatment during very early pregnancy decreases the number of live fetuses in UHO gilts, suggesting that uterine capacity was decreased. However, this interpretation should be made with some caution. Other possible explanations for these results are that the treatments interfered with fertilization rate, oviductal transport, or early embryonic survival.