The passive compliance and resistance of the respiratory system were measured in 12 spontaneously breathing newborn infants before and after endotracheal extubation. End-inspiratory airway occlusions were used to relax the respiratory muscles, allowing occlusion pressure to be measured and respiratory system compliance and resistance to be calculated from the flow volume relationship of the subsequent passive expiration. Airway pressure was measured from an endotracheal tube or a face mask, expiratory flow from a pneumotachograph, and expiratory volume from the integrated flow signal. In six of the infants, diaphragmatic electromyography was also performed before and after extubation. Resistance and EMG findings were both decreased by extubation (mean decrease 43.9%, P less than 0.001 and 27.3%, P less than 0.05, respectively), but compliance was unchanged. Thus, by substantially increasing resistance, an endotracheal tube causes the diaphragm to increase its activity to maintain ventilation.