We determined the effect of heparin pretreatment in modifying the pulmonary hypertension and reduced pulsatility of pulmonary capillary blood flow (\(\dot Q_c\)) wave observed after thrombo-embolization in dogs. In vivo thrombi were produced in 24 dogs, 12 of which were pretreated with heparin. Pulmonary arterial pressure and\(\dot Q_c\), using the body plethysmograph-nitrous oxide method, were measured before and after the release of emboli. Alteration in pulsatily of instantaneous\(\dot Q_c\) were quantitated by changes in the peak-to-mean flow ratio (\(\dot Q_c\) max/\(\bar Q_c\)), the ratio of peak flow to stroke volume (\(\dot Q_c\) max/SV) and the percentage of stroke volume flowing through the capillaries during systole (Vsyst/SV). Mean\(\dot Q_c\) did not change significantly after embolization in the control (Group I) or heparinized animals (Group II). Mean pulmonary arterial pressure increased 70 per cent in mild embolism of Group I and 240 per cent in severe embolism. In Group II corresponding increases were 49 per cent and 115 per cent. In Group I,\(\dot Q_c\) max/\(\bar Q_c\),\(\dot Q_c\) max/SV and Vsyst/SV decreased significantly during embolic period of mild and severe embolism. There was no significant change in pulsatility in Group II when five or less segments of second tributory of pulmonary artery were obstructed. With more diffuse embolization the reduction on pulsatility was similar to Group I. Transient pulmonary hypertension and decreased\(\dot Q_c\) pulsatility with minimal or moderate vascular obstruction by emboli indicate diffuse precapillary vasoconstriction. This is modified but not prevented by heparin, depending on the extent of embolization.