1. (1) Seven patients with mitral insufficiency and giant V waves in the wedged pulmonary artery position have been studied. 2. (2) The systolic peak of the V (or regurgitant) wave approximated the systolic peak of the right and main pulmonary artery pressure curves. 3. (3) Pulmonary arteriolar resistance was found to be normal or reduced. 4. (4) A consideration of the hemodynamic data favors the concept that in mitral insufficiency a retrograde transmission of the V wave may occur and that it results from an increase in the tone of the pulmonary vascular system without an increase in the pulmonary vascular resistance.