text: The proposed research is aimed at investigating the inhomogeneity of gas mixing and ventilation distribution in the lungs of normal subjects and patients with various types of airflow obstruction. The research will focus on the concentration differences between units ventilated in parallel within small lung regions as distinct from interregional, topographically distributed differences. A major objective will be to use the separation of inert tracer gases of different diffusivity as an indicator of intraregional inhomogeneity and thus to partition the measure of inequality of ventilation made at the mouth into interregional and intraregional components. An alternative method will be to use gamma ray emission of 133 Xenon to measure intraregional inhomogeneity of ventilation and its regional distribution in differnt postures, by analyzing the curvilinearity of regional washouts of 133 Xenon. The research will also include the quantitation of the contribution of cardiogenic gas mixing to gas exchange in dogs both during quiet breathing and in breathholding. Possible mechanisms of cardiogenic gas mixing will be analyzed using a model of gas transport by simulating observed patterns of gas mixing in dogs.