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The pattern of rise in progesterone during the early es-trous cycle and pregnancy has been examined in numerous experiments. Measurable increases in progesterone concentrations in the peripheral plasma are typically observed by Day 3, and luteal phase levels (15-25 ng/ml) are reached by about Day 6. The dose of progesterone used in the current experiment has also been used in numerous studies to mimic progesterone concentration during the luteal phase. However, it is difficult to predict how much earlier the progesterone treatment used in this study triggers the timing mechanism for uterine protein secretion and conceptus development, primarily because the threshold for this trigger is not known. From the results of Vallet et al., who used a treatment regimen identical to that used here, one can estimate an acceleration of about 24 h, which is consistent with the hypothesis that the low but measurable levels of progesterone present on Day 3 of the estrous cycle and pregnancy are sufficient to trigger progesterone timing. This finding is also consistent with the effect of mifepristone treatment on Day 2, because the halflife of this drug would prevent significant interference with progesterone beyond about Day 3. However, further studies are needed to establish the threshold concentration of progesterone necessary to trigger the progesterone timing mechanism.