The effects of nitroglycerin, dopamine and dobutamine on pulmonary gas exchange were determined in 21 adult patients during two-lung and one-lung ventilation. Nitroglycerin, in 1 microgram.kg-1.min-1, decreased cardiac index (CI) and PaO2 during both two- and one-lung ventilation, and increased in Qs/Qt during one-lung ventilation. There were no significant changes in the measured variables during infusion of dopamine, 5 microgram.kg-1.min-1. Dobutamine, 5 microgram.kg-1.min-1, increased CI and PaO2 did not change during two-lung ventilation. During one-lung ventilation, PaO2 increased from (mean value +/- SD) 168 +/- 46 to 201 +/- 52 mmHg (P less than 0.01) with dobutamine infusion. Qs/Qt decreased from 29.2 +/- 7.0 to 26.0 +/- 6.2 per cent (P less than 0.05) without any change in pulmonary vascular resistance index during one-lung ventilation. We conclude that dobutamine has advantages over dopamine and nitroglycerin during one-lung ventilation.