The possible influence of sympathoadrenal activity on peripheral blood flow was studied. Limb blood flow was measured with venous occlusion plethysmography at 30 minutes, 2 hours, and 24 hours after birth in 24 healthy, full-term infants, of whom 14 were delivered by elective cesarean section. Mean arterial pressure was simultaneously measured noninvasively, and peripheral vascular resistance calculated. Umbilical artery blood at birth and peripheral venous blood at 2 and 24 hours were analyzed for concentrations of catecholamines and hematocrit. The limb blood flow was significantly lower at 30 minutes in the vaginally delivered infants compared with those delivered by cesarean section. There was a gradual increase in limb blood flow over the 24 hours in those delivered vaginally, whereas in the section group only small changes were observed. The peripheral vascular resistance was higher both at 30 minutes and at 2 hours in those delivered vaginally, which corresponded to the higher catecholamine concentrations at birth and at 2 hours in this group. Two hours after birth there was a significant correlation between noradrenaline levels and peripheral vascular resistance. The results indicate that the sympathoadrenal system influences peripheral circulation at birth and is important in the circulatory adaptation of the newborn infant.