The norepinephrine concentration in 77 umbilical, arterial, and venous plasma samples, and in 31 simultaneous maternal plasma samples, was measured. The mean (±SEM) of 8.7 ±1.9 ng/ml in the umbilical artery was higher (P < .001) than that of 3.6 ± 0.9 ng/ml in the umbilical vein. In paired fetal-maternal venous samples the norepinephrine concentration of 3.8 ±1.7 ng/ml in the fetus was higher (P < .05) than that of 0.3 ±0.1 ng/ml in the mother. Among the different types of vaginal deliveries the umbilical arterial norepinephrine concentrations were: 5.8 ±2.1 ng/ml in uncomplicated vaginal deliveries; 16.4 ±2.1 ng/ml in breech deliveries (P < .05 as compared with uncomplicated vaginal deliveries); 8.8 ±2.5 ng/ml in vacuum extraction deliveries; and 0.8 ±0.3 ng/ml and 11.3 ±7.7 ng/ml in twin A and twin B deliveries, respectively. All these values were higher (P < .001) than those after elective cesarean section, except that for twin A, which was lower (P < .01) than that for twin B, indicating that labor and vaginal delivery induced activation of the fetal sympathoadrenal system. (Obstet Gynecol 61:19, 1983)