The immediate postnatal metabolic adaptation and sympatho-adrenal activation were studied in infants delivered vaginally or by elective caesarean section. Vaginally delivered infants showed high catecholamine levels at birth compared to infants born by caesarean section under epidural or general anaesthesia. Umbilical arterial glucose levels were significantly higher in the vaginal group than in both caesarean section groups. At 30 min, all groups showed a marked decrease with several infants showing asymptomatic hypoglycaemia in the caesarean section group. C-peptide levels showed no difference at birth but later became significantly higher in the vaginal group. Although the levels of free fatty acids and glycerol were low at birth, they were significantly higher in the vaginal group. In all groups they increased substantially with time. Considering the marked differences in catecholamine levels, the differences in metabolic adaptation were unexpectedly small. This implies an attenuated metabolic response to sympatho-adrenal stimulation in the newborn.