In order to throw light upon the controversial mechanism of pulmonary hypertension associated with hypertensive shock due to acute pulmonary embolism, the following experiments were carried out. Strictly unilateral pulmonary embolization with Lycopodium spores was attempted in the dog, and the results were compared with those obtained by unilateral balloon occlusion of the ipsilateral pulmonary artery as the control. In addition, reccurrent embolizations with Lycopodium spores were also performed to observe a critical point at which lethal systemic hypotension would occur. Method s A suspension of Lycopodium spores in saline was prepared so as to be a mixture of 50 mg/ml. The adult mongrel dog was anesthetized with pentobarbital soda (an initial dose of 25 mg/kg) and heparinized. 1. A balloon tip catheter with an end hole was placed in the proximal left pulmonary artery. Effects of balloon occlusion of the left pulmonary artery on the pulmonary artery pressure was compared with that of strictly ipsilateral Lycopodium embolization, in the same animal. In order to evaluate the degree of obstruction of the blood flow, end-tidal CO2 concentration from the left lung was continuously monitored employing a bronchospirometric tube. The systemic and pulmonary artery pressures were consistently recorded. To confirm that Lycopodium spores had not overflowed into the contralateral lung, a smear preparates obtained by means of incising of the right lung lobes were microscopically checked very carefully, in postmortem examination. 2. An initial dose of 100 to 200 mg of Lycopodium spores was injected into the right atrium, then a dose of 25 to 100 mg was added every 5 to 15 minutes in the same manner. In this series, the changes in the pulmonary and systemic artery pressures and the respiratory rate were observed. Results When strictly left pulmonary artery embolization was accomplished, the elevation in the pulmonary artery pressure never exceeded that observed in the control balloon occlusion of the ipsilateral pulmonary artery. In this group, the elevation in the pulmonary artery pressure by superimposed balloon occlusion after embolization was as same in the degree as by the control occlusion. No or negligible overflowing of Lycopodium spores into the right pulmonary artery was confirmed by the postmortem examination (10 experiments).