The ontogeny of functional sympathetic neural, adrenal medullary, and extra-adrenal components of adrenergic control of heart rate was examined in neonatal rats using a combination of pharmacological and surgical approaches. In the first experiment, bradycardia (-54 beat/min) was observed in unrestrained 2-day Sprague-Dawley rat pups 2.5 h after treatment with adrenergic neuron blocking agent bretylium tosylate, indicating that sympathetic neuronal control of basal heart rate is functional in newborn rats. (Two additional experiments examined the possibility that bretylium reduced heart rate through a nonspecific cardiodepressant action). Heart rate decreased further after administration of the ganglionic blocking agent hexamethonium to 2-, 5-, 8-, and 24-day pups pretreated with bretylium and methylatropine, indicating that neurally controlled release of catecholamines from the adrenal medulla contributes to maintenance of basal heart rate in preweanling rats. In pups pretreated with bretylium, methylatropine, and hexamethonium, additional bradycardia was observed after administration of the β-adrenergic receptor blocking agent atenolol, suggesting that circulating catecholamines from extra-adrenal sources (e.g., organ of Zuckerkandl) are important in control of heart rate during the neonatal period. A second experiment using adrenalectomized 2-, 8-, and 24-day rats confirmed the relative contribution of adrenal catecholamines to sympathetic control of neonatal heart rate. These results suggest that sympathetic innervation of both the heart and adrenal exerts significant functional control of cardiac rate in newborn rats. Circulating catecholamines released from extra-adrenal chromaffin tissue may also provide significant adrenergic stimulation to the heart during the neonatal period.