I n order to evaluate separately changes in vascular tone occurring in arteries and veins, we measured pulmonary capillary red blood cell (RBC) concentration under zone II (waterfall) conditions in isolated dog lungs rapidly frozen with Freon 12. The lungs were frozen while being perfused from artery to vein and from vein to artery breathing normal and hypoxic gas mixtures and during infusions of serotonin and histamine. Changes in capillary RBC concentration which occurred during the experimental conditions indicated an alteration in vascular resistance upstream from the capillaries. Alveolar hypoxia caused a significant decrease in capillary RBC concentration during forward perfusion, but no change from the control values during reverse perfusion. Serotonin infusion caused a decrease in RBC concentration during forward perfusion comparable with that of hypoxia and a small but significant decrease during reverse perfusion. Histamine infusion caused no change in RBC concentration from control values during forward perfusion, but a large decrease during reverse perfusion. We conclude that vasoconstriction occurs (a) exclusively in arteries during alveolar hypoxia, (b) predominantly in arteries but to a lesser extent in veins during serotonin infusion, and (c) exclusively in veins during histamine infusion.