Estrone, 17β-estradiol and estriol were estimated fluorometrically in serial samples of maternal plasma from 5 subjects in late pregnancy, during labor and post partum. Estradiol concentrations were stable from 2 to 3 weeks prior to delivery until placental delivery, when significant decreases occurred. While significant increases and decreases in estrone and estriol did occur during late pregnancy, no consistent pattern was observed. Levels in early labor were not significantly different from those during the final week of pregnancy. In the majority of subjects significant decreases did not occur until placental delivery. Rising estrogen levels were observed during oxytocin-induced labor, after premature rupture of membranes, in a single patient. It is suggested that a stable hormonal environment may be a prerequisite for initiation of labor.