Long-term intermittent mechanical ventilation results in improvements in ventilatory performance and clinical status between ventilation sessions in patients with chronic respiratory failure. The application of intermittent positive pressure ventilation through a nasal mask (NPPV) is a simple, noninvasive method for the provision of chronic intermittent ventilatory support. We investigated the effects of NPPV on inspiratory muscle activity in three normal subjects and nine patients with acute or chronic ventilatory failure due to restrictive (four subjects) or obstructive (five subjects) respiratory disorders. NPPV resulted in reductions of phasic diaphragm electromyogram amplitude to 6.7 +/- 0.7 percent (mean +/- SEM) of values obtained during spontaneous breathing in the normal subjects, 6.4 +/- 3.2 percent in the restrictive group, and 8.3 +/- 5.1 percent in the obstructive group. Simultaneous decreases in activity of accessory respiratory muscles were observed. The reductions in inspiratory muscle activity were confirmed by the finding of positive intrathoracic pressure swings on inspiration in all subjects. With NPPV, oxygen saturation and PCO2 remained stable or improved as compared with values obtained during spontaneous breathing. These results indicate that NPPV can noninvasively provide ventilatory support while reducing inspiratory muscle energy expenditure in acute and chronic respiratory failure of diverse etiology. Long-term assisted ventilation with NPPV may be useful in improving ventilatory performance by resting the inspiratory muscles.