The effect of unilobar engorgement and edema in a dog's lung on the effort of breathing was studied by expressing the latter as the velocity of air-flow attained per unit of pressure change across the lung, and as static compliance. Edema of the left lower lobe, produced by shunting blood from the femoral into the pulmonary artery, affected the effort of breathing only at high infusion rates and volumes, otherwise changes in resting (mid) position and respiratory rate kept the effort of breathing constant. Static compliance decreased with infusion, possibly because the infused blood encroached on the air space.