One hundred and seventy-two plasma progesterone determinations were performed in normal pregnancy, labor, and the puerperium. One hundred and nineteen of the assays were done on isolated samples; 53 of the assays were done in 11 serial studies. While the concentration of progresterone in peripheral blood increased progressively with advancing pregnancy, no apparent decrease in the hormone level occurred at or near the onset of labor or during its progress. A significant decrease in plasma progesterone one hour after the delivery of the placenta continued to decline to menstrual cycle levels by 72 hours post partum.