Distributions of ventilation and perfusion relative to Va/Q were determined in seven young healthy volunteers (24-33 yr) while they were either in the supine or right lateral decubitus position. The subjects were studied first awake and then while anesthetized-paralyzed and breathing 30% oxygen and again while breathing 100% oxygen. In the awake state, no statistically significant differences were observed in the distribution of ventilation and perfusion relative to Va/Q between the supine and right lateral decubitus positions or on changing the inspired oxygen concentrations. After induction of anesthesia-paralysis, Va/Q mismatching increased significantly but only small right-to-left intrapulmonary shunts developed. Ventilating the lungs with 100% oxygen further increased the dispersion of blood flow distribution during anesthesia-paralysis; lung units with low Va/Q or right-to-left intrapulmonary shunts (or both) developed. With induction of anesthesia-paralysis and intubation of the trachea, the anatomic dead space was decreased and the alveolar dead space increased.