ExtractIn order to determine whether the mechanical characteristics of the lungs of newborn infants imposed limitations on their ventilatory response to carbon dioxide, 11 premature and 5 full term infants ranging in gestational ages from 28‐40 weeks and in birth weights from 964 to 4,040 g were studied by a rebreathing technique. Sensitivity to carbon dioxide determined by this method revealed a good correlation with compliance (r = 0.6,P< 0.001), and an even stronger correlation with gestational age (r = 0.84,P< 0.001). Because compliance also improved with increasing postconceptional age, the current study does not permit one to differentiate between neurologic or pulmonary mechanical factors which may affect ventilatory response to carbon dioxide.SpeculationInfants who respond to carbon dioxide with an increase in minute ventilation should also increase their rate of pulmonary work. Failure of this work rate to increase in response to elevated carbon dioxide tensions may indicate that nonresponsive infants are already working at their maximum level of pulmonary work, and would thus permit differentiation of infants who have mechanical limitations to increased performance as opposed to those who lack chemoreflexes.