Pressure curves in the pulmonary vein and in the left auricle were recorded in patients with stenosis of the left atrioventricular valve before and after mitral valvotomy. The curves showed that the sphincters of the pulmonary veins may fail when the mean pressure in the left auricle exceeds 20 mm Hg. Normally, by blocking the reverse flow, the sphincters protect the pulmonary vessels from the back pressure wave. With increased pressure in the left auricle, insufficiency of the pulmonary vein sphincters could provoke changes in the small pulmonary vessels, which were demonstrated histologically on biopsy material, and which had the effect of increasing the resistance to blood flow. Auricular fibrillation, by causing the sphincters to become ineffectual even when the hypertension in the left auricle is moderate, may promote a relatively early rise of pulmonary vascular resistance.