Concentrations of estrone, estradiol, and progesterone in the myometrium, maternal peripheral vein, and mixed umbilical cord plasma were determined before and during labor by radioimmunoassay. Of the 29 patients studied, 4 underwent elective abortions by hiysterotomy in the second trimester of pregnancy, and 25 underwent cesarean section (elective or during labor). The concentration of umbilical cord Cortisol in the latter group of patients was also determined. Advanced labor was characterized by a high concentration of estrone in the maternal peripheral vein, the umbilical cord, and the myometrium. Concentrations of estradiol, progesterone, and Cortisol in cord plasma also increased during labor. From the second trimester to term pregnancy, increases in the concentrations of estrone and estradiol in the myometrium were proportionately greater than those of progesterone. During labor, there were no significant changes in the myometrial concentrations of estrone, estradiol, or progesterone. During labor in term pregnancies, the myometrial concentration of estrone was higher than that of estradiol.