Activity from afferents of respiratory origin was recorded from filaments of the vagus nerve in puppies 1-17 days old. Animals were anesthetized with a mixture of chloralose-urethane and artificially ventilated with the chest open. A total of 256 receptors were studied of which 96% were identified as slowly adapting mechanoreceptors (SARs) and 4% as rapidly adapting mechanoreceptors (RARs). Hence the newborn has fewer spontaneously active RARs than the adult. Thirty-five percent of SARs were located in the trachea and the remainder in the bronchi: a distribution similar to that found in the adult. The transpulmonary pressure (Ptp) threshold required for SAR activation was higher in the newborn than the adult for both tracheal (2.42 cm H2O vs 0.51 cm H2O) and bronchial 4.66 cm H2O vs 3.66 cm H2O) receptors. This resulted in less SAR activity at FRC in the puppy (16% of the total number of SARs in 1-17-day-old animals) compared to the adult (63%). In addition, the discharge frequency of tracheal and bronchial SARs, at any given Ptp, was lower in the newborn than in the adult dog. These data suggest that the newborn has considerably less volume-related feedback during the respiratory cycle compared to the adult.