Blood pressure and heart rate were recorded in 15 anesthetized puppies (6-10 wk, 1-6 kg) and 18 adult mongrel dogs (>1 yr, 18-26 kg) before and during acute blood pressure changes achieved with nitroglycerin or phenylephrine (4 and 8 μg/kg iv). Overall heart rate responses to blood pressure changes in adults were significantly (P < 0.05) greater than those in puppies. Following control baroreflex responses, two multifiber efferent preparations from the discrete thoracic cardiac nerves (sympathetic, n = 48; parasympathetic, n = 18) were simultaneously recorded and analyzed by microprocessor. Severing of the nerves signfiicantly attenuated the heart rate responses to blood pressure changes in puppies only, suggesting less redundancy of the neural regulation of the sinus node in the puppy. The pressure-induced reflex changes in the sympathetic or parasympathetic efferent nerve activities were not significantly different between adult dogs and puppies. There were no significantly differences in reflex activities in right-sided (n = 29) vs. left-sided (n = 19) sympathetic nerves in either puppies or adult dogs. Preganglionic sympathetic fibers in puppies (but not adult dogs) were more responsive to blood pressure changes than were postganglionic sympathetic fibers. Thus baroreceptor reflex control in the puppy is less developed than in the adult canine heart, and the maturational difference in neural regulation of the heart is at or beyond the efferent nerve terminals.