Distribution of ventilation and perfusion in relation to ventilation-perfusion ratio (&OV0312;A/&OV0312;) were studied in 14 patients, with a mean age of 59 yr, before elective lung surgery, in the supine position when awake, during intravenous anesthesia and mechanical ventilation with air, after increasing the fraction of inspired oxygen (FI02) to 0.5, and in the lateral position. Before anesthesia, small inert gas shunts and perfusion of low &OV0312;A/&OV0312; regions, indicating some degree of &OV0312;A/&OV0312; mismatch, were observed in several patients. After induction, FIo2= 0.21, the major changes were a significant decrease in cardiac output and an increase in log SD for perfusion from 0.77 ± 0.45 (SD) to 1.13 ± 0.50 (SD), while the shunt remained low at 1% of cardiac output and arterial oxygen tension (PaO2) was unchanged. An increase to FIO2= 0.5 induced only small changes with a shunt of 2.5% of cardiac output. In the lateral position, the shunt was 4.0% and increases in ventilation to high &OV0312;A/&OV0312; regions were observed. The lack of marked changes in the &OV0312;A/&OV0312; distribution after induction either could be a result of only minor alterations in the distribution of ventilation and perfusion or an effective vascular response to alveolar hypoxia (hypoxic pulmonary vasoconstriction, HPV).