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 less than .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 less than .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 less than .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 less than .001) than those after elective cesarean section, except that for twin A, which was lower (P less than .01) than that for twin B, indicating that labor and vaginal delivery induced activation of the fetal sympathoadrenal system.