Conventional Starling heart -lung preparations were used to ascertain the presence or absence of pulmonary AV shunts. Lucite spheres of 75-80 u introduced into the pulmonary arteries of these preparations under physiologic pressures passed from the arteries to the pulmonary veins. Larger spheres, 300 u in diameter, did not traverse the lung vasculature. It is concluded that small AV shunts are extant in the lungs of dogs.