Thirty-three patients who required short-term postoperative mechanical ventilatory support were studied to compare different criteria established to initiate weaning from mechanical ventilation. Intermittent mandatory ventilation criteria (i.e., decreasing mechanical respirator rate as long as the arterial (pHa) remains above 7.35) and conventional criteria (ie, vital capacity greater than 15 ml/kg and peak negative pressure greater than 20 cm H2O) were compared to determine which would more rapidly predict a patient's ability to sustain total spontaneous respiration. All patients were eventually weaned from mechanical ventilation and had their tracheas extubated. Twenty-one patients maintained a pHa of greater than 7.35 during total spontaneous ventilation before they would, or could, meet conventional criteria for initiating a trial of spontaneous respiration (P less than .001). Seven patients simultaneously met both criteria for maintaining total spontaneous ventilation and the remaining five patients met conventional criteria before intermittent mandatory ventilation criteria. In the latter group, the pHa decreased below 7.35 during spontaneous respiration but in only one patient did it fall below 7.30. Our findings suggest that a patient's ability to maintain a pHa of greater than 7.35 while decreasing the frequency of mechanical ventilator breaths is more accurate than peak negative pressure and vital capacity for predicting ability to sustan adequate spontaneous respiration.