Bronchial resistance was measured by the lung overflow technique in guinea pigs. Intravenous infusion of thrombin produced a significant increase of bronchial resistance, induced by platelet constituents released from the thrombi and deposited in the arteries and capillaries of the lung. This was evidenced by increase of bronchial resistance and mortality having been aggravated by e-aminocaproic acid (inhibition of fibrinolysis), by infusion of gelatin (sludge phenomenon) as well as by catecholamines (stress effect) and significantly inhibited by massive doses of heparin and by experimental thrombocytopenia. Emphasis is laid on the significance of pulmonary microembolism in the development of severe pulmonary hypertension and edema, the cause of which is difficult to diagnose clinically, yet demonstrable at autopsy. The recommended method is held to be suitable for the objective study of the process in animal experiment.