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Results of experiment 3 supported this hypothesis. For gilts treated with the 400mg dose of mifepristone, both conceptus development and uterine protein secretion were lower in the mifepristone-treated gilts compared with controls. Because these results are qualitatively similar to previous results obtained using Meishan gilts, the effect of this treatment on uterine capacity was examined. Unexpectedly, uterine capacity was also decreased in the mifepristone-treated gilts compared with controls. Unlike the effect of early progesterone treatment, mifepristone resulted in fewer small conceptuses (both small fetuses and small placentas) in the mifepristone-treated gilts. These results suggest that both too early a rise in progesterone and too late a rise in progesterone during early pregnancy are associated with decreased uterine capacity and that an optimum pattern of rise in progesterone concentrations likely exists during this period. The 400-mg dose of mifepristone reduced both uterine protein secretion and conceptus size by about 50%, which is consistent with the hypothesis that progesterone concentrations during early pregnancy influence the timing of both of these events. However, the stimulatory effect of the 100mg dose was unexpected and is more difficult to explain.