The respiratory response to therapeutic lung inflation by artificial ventilation in premature babies has been studied. Both the Hering-Breuer and an augmented inspiratory reflex could be provoked by ventilator inflation in neonates as premature as 25 weeks gestational age. No chemo-receptor control was demonstrated on the presence of these reflexes. Augmented inspirations were only seen in the first five postnatal days regardless of gestational age, but the Hering-Breuer reflex persisted throughout the period of study of 11 days. The frequency of elicitation of augmented inspirations was shown to be inversely related to the dynamic compliance. The relative stimulation of these two respiratory reflexes may explain why some infants are easy to ventilate and others difficult.