The effects of occlusion of the left anterior descending coronary artery on pulmonary hemodynamics and calculated pulmonary vascular resistance were measured in 17 open chest dogs anesthetized with chloralose. Coronary occlusion was associated with a slight but insignificant increase in mean pulmonary arterial pressure together with a significant elevation in mean left atrial pressure (P less than 0.01) and a significant reduction in mean pulmonary flow (P less than 0.01). Calculated pulmonary vascular resistance was significantly reduced during occlusion (P less than 0.01). The reduction in resistance was associated with elevated mean left atrial pressure and therefore reflected passive distension of pulmonary vessels secondary to suppressed left ventricular function. In contrast, coronary occlusion after vagotomy was not associated with a significant reduction in pulmonary vascular resistance, and in six experiments the decrease in resistance was converted to an increase. It is concluded that, before vagotomy, pulmonary hemodynamics during coronary occlusion are primarily affected through passive vessel distension whereas, after vagotomy tonic suppression of sympathetic activity is interrupted, thereby allowing enhancement of sympathetic vasoconstrictor activity to the pulmonary vascular bed and producing an increase in calculated resistance.