A noninvasive technique for determining cardiac output, lung blood-tissue volume, and the volume of perfused body tissue in shock states has been developed from analysis of the pulmonary washout of several physiologically inert tracer gases. Canine preparations and a computer-based simulation were used to evaluate the accuracy and feasibility of the technique. The dogs were ventilated with 10% helium, 10% N2O, 0.45% C2H, 39.5% N2, and 40% O2 mixture, and washed out with 60% N2 and 40% O2. End-tidal volume excretion differences between the three inert gases helium, N2O, and C2H2 resulted from differences in the blood/gas partition constants (Λ) of the test gases. Helium, which has a very low Λ, is contained in lung air space, whereas N2O and C2H2, with moderate Λ's, are distributed to lung air space, blood, and perfused tissues. Computer modeling of the cardiopulmonary and body perfusion gas transport permitted calculation of alveolar ventilation, V̇A/𝑄̇ć disparity, pulmonary blood-tissue volume, pulmonary blood flow, V̇A/𝑄̇ć, and perfused body volume in the canine experiments. In experimental shock settings and in a variety of control situations over an eightfold range of flows the pulmonary blood flow calculated from the N2O and C2H2 washout was generally within ±15% of the cardiac output determined by the cardiogreen technique.