The oxygen consumption of rat bronchioles suspended in a physiological salt solution containing plasma proteins was measured with the Cartesian diver microrespirometer. The oxygen consumption of the bronchiolar tissue 100-300 and 300-400 μm in diam was significantly (P<0.01) reduced as the protein concentration of the suspending solution was increased from the low to the high extreme of the normal plasma physiological range and as the diffusion distance through the suspending medium was increased from 25 to 50 μm. The reduction in the bronchiolar oxygen consumption was significantly (P<0.01) reversed by using 95% oxygen-5% nitrogen instead of air as the diffusing gas in the Cartesian diver. Measurements of the oxygen diffusivity in the protein solutions using a diaphragm diffusion cell showed a large decrease in the diffusivity as the plasma protein concentration was increased over the same concentration range used in the oxygen consumption studies. These results suggest that the reduction in oxygen consumption was secondary to a decrease in oxygen diffusion and may provide at least a partial explanation for the diffusion abnormalities which exist in noncardiogenic pulmonary edema in which there is an increase in microvascular membrane permeability to proteins.