Distribution of ventilation (V) and perfusion (Q) was studied with 133Xe in eight supine, normal subjects comparing spontaneous breathing (SB) and intermittent positive-pressure breathing (IPPB). Tidal volume, inspiratory flow, and breathing frequency measured during SB were closely matched during automatically triggered IPPB. V and Q in the lung bases (adjacent to the diaphragm) were decreased relative to other regions during SB and further diminished by IPPB at similar volumes. During IPPB, basilar V and Q improved when tidal volume was increased; however; spontaneous hyperinflation resulted in significantly higher basilar V and Q than large tidal volumes delivered by IPPB. Thus, changes in lung volumes and gas exchange in the supine posture are attributable to impaired V and Q in the bases but not in dependent (posterior) regions. IPPB further reduces basilar V and Q, possibly due to loss of interdependence resulting from diminished respiratory muscle contraction. These findings may explain atelectasis during prolonged IPPB in supine patients. Although large tidal volumes improve basilar V during IPPB, spontaneous deep breaths are more effective and may prevent atelectasis better than IPPB at similar tidal volumes.