A new method for determining the distribution of ventilation-perfusion ratios in critically ill patients in the intensive care setting is described. The method is based on the infusion of a mixture of inert gases into the venous circulation with a subsequent measurement of the gas concentrations in arterial blood and mixed expired gas by gas chromatography. Young normal subjects show very narrow distributions of ventilation and blood flow with respect to ventilationperfusion ratio. These distributions broaden with increasing age. Oxygen breathing causes the development of shunts in both normal and abnormal lungs, particularly in patients with lung disease. The mechanism of this is discussed. Measurements of shunt after 100% oxygen generally result in overestimates. In some abnormal lungs, positive end-expiratory pressure decreases the amount of shunt and also results in increased amounts of ventilation going to units with very high ventilation-perfusion ratios. Preliminary results suggest that the new test may be of diagnostic value in some settings; eg, detecting pulmonary embolism in critically ill patients.