The washout of a bolus of helium (inhaled from residual volume) during relaxed expiration (RE) through a resistance was compared with that during expiration with voluntary diaphragmatic contraction (VDCE) achieved by tensing the abdominal muscles while maintaining constant expiratory flow (less than 0.4 l/s). In six subjects in the lateral decubitus position, phase IV during RE started at 58 +/- 1.6% vital capacity (VC) (mean +/- 1 SE). During VDCE, when the transdiaphragmatic pressure (Pdi) exceeded 30 cmH2O, phase IV commenced at 8.2 +/- 0.8% VC. The expired He concentration (FEHe) at lung volumes greater than 70% VC was 1.3 +/- 0.05 times that during RE. In supine subjects VDCE flattened the slope of the alveolar plateau and decreased closing volume by 3.7 +/- 0.4% VC. Our results suggest that when the diaphragm is relaxed in the horizontal subject, the hydrostatic gradient of pressure within the abdomen enhances early emptying of dependent lung zones. Diaphragmatic contraction results in more homogeneous emptying. We conclude that diaphragmatic tone influences the vertical gradient of regional volume, and hence of pleural pressure. The latter is not constant, being less during inspiration than during expiration. Therefore, differences in the changes of applied pressure between dependent and nondependent lung regions influence ventilation distribution in subjects in the horizontal posture.