Plasma progesterone concentration was measured serially in 32 women during the last 10 wk of pregnancy. There were 18 primigravidae and 14 multiparae. The mean progesterone levels increased gradually from 101 ng/ml at week 30 to 168 ng/ml at week 39 and fell slightly to 165 ng/ml at week 40 of gestation. The mean progesterone levels calculated in relation to the day of spontaneous onset of labor. A linear rise in plasma progesterone levels reached a plateau during the last 2 wk preceding labor. The slight drop in progesterone level during the week before onset of labor was not statistically significant.