Rapid injections of hypertonic sucrose, sodium chloride, and urea solutions were given in the jugular veins of anesthetized dogs, and samples of blood were collected from the aorta. Following each of these injections, hemoglobin and plasma protein concentrations in the collected blood declined because fluid was extracted from the lungs. Injections of sucrose and sodium chloride, but not of urea, caused the hemoglobin concentration to fall more than the plasma protein concentration and slowed the movement of labeled red cells relative to that of labeled albumin when these two substances were injected at the same time and site as were the hypertonic solutions. The net flux of plasma through the lungs exceeded that of red cells for several seconds, indicating that plasma might pass around the more slowly moving red cells and through the smaller vessels of the lung. Impairment of red cell movement through the lungs is attributed primarily to red cell dehydration and loss of deformability. Unlike sucrose and sodium chloride, urea rapidly equilibrates between red cells and plasma and can only transiently dehydrate red cells. No impairment of red cell movement was seen after injections of hypertonic solutions of urea.