The maturity of beta-adrenoceptors in newborn infants was studied in relation to the catecholamine surge during labor. Umbilical blood was collected at birth from 12 infants delivered vaginally and 13 infants delivered by elective cesarean section. Granulocytes and lymphocytes were isolated. Receptor numbers and binding affinity were determined in the granulocytes by incubation with 125I-iodohydroxybenzylpindolol. Receptor responsiveness was tested by assessing isoproterenol-induced cyclic AMP accumulation in lymphocytes. Significantly higher plasma noradrenaline, adrenaline, and dopamine concentrations were found in infants born vaginally (108; 8.9; 0.9 nmol/liter, liter, respectively, median values) as compared with those delivered by cesarean section (11.0; 2.4; 0.2 nmol/liter). No significant differences in beta-adrenoceptor binding sites (receptor number: 39.2 +/- 2.6 versus 44.7 +/- 5.9 fmol/mg protein and binding affinity: 66.6 +/- 7.8 versus 65.0 +/- 6.2 pM) or responsiveness (maximal isoprenaline induced cAMP formation 52.4 +/- 10.3 versus 40.6 +/- 8.9 pmol/10(6) cells) were found between the two groups of infants. Lymphocyte beta-adrenoceptor sensitivity was similar to that found in adults. The beta-adrenoceptors on whole blood cells seem to be mature at birth and have the same responsiveness as in adults. The higher catecholamine surge during vaginal delivery as compared to elective cesarean section does not seem to affect beta-adrenoceptor function. Our results do not support the idea that reduced beta-adrenoceptor function is the cause of the previously observed inappropriately small cardiovascular and metabolic responses to the exceptionally high plasma catecholamine concentrations at birth.