During a four-month period, all babies who received mechanical ventilation in the Neonatal Intensive Care Unit were studied to determine the effects of artificial ventilation on spontaneous respiratory activity. The babies were either totally apneic or ventilator inflation stimulated one of four distinct spontaneous respiratory patterns: synchronous breathing, Hering-Breuer reflex, augmented inspiration, or active expiration against ventilator inflation. The particular interaction evoked was dependent on the frequency of ventilation and the clinical condition of the baby. Only one pattern, active expiration against ventilator inflation, was consistently recorded before the development of pneumothorax. Preliminary evidence indicates that immediate paralysis of the baby as soon as that pattern is demonstrated may prevent the occurrence of pneumothoraces.