The effects of unilateral hypoxic ventilation on pulmonary hemodynamics, alveolar-arterial oxygen tension difference (A-aDO2) and intrapulmonary shunt in 10 dogs were studied under unilateral hypoxic ventilation using nitrogen. Hear rate, mean arterial pressure, central venous pressure, mean pulmonary arterial pressure, pulmonary capillary wedge pressure, cardiac output, blood gases and hemoglobin were measured during controlled ventilation with 100% oxygen and unilateral hypoxic ventilation. Using the above steps pulmonary hemodynamics, (A-a)DO2 and intrapulmonary shunt were calculated and the following results were obtained. 1) Mean pulmonry arterial pressure significantly increased (20%) from 20.6+/-2.60mmHg to 24.8+/-2.46mmHg(p<0.005) and pulmonary vascular resistance also significantly increased(43%) from 434+/-68.4 dynee. sec/cm(2) to 605+/-81.0 dynes. sec/cm(2)(p<0.005), whereas the cardiac output decreased (8%) from 1.92+/-0.23 l/min to 1.76+/-0.21 l/min(p<0.05) after unilateral hypoxic ventilation. 2) Alveolar-arterial oxygen tension difference significantly increased from 180+/-23.2mmHg to 470+/-31.9mmHg(p<0.005) after unilateral hypoxic ventilation. 3) Intrapulmonary shunt significantly increased from 9.5+/-1.40% to 24.8+/-2.02%(p<0.005) after unilateral hypoxic ventilation, but it was much lower than the mathematically expected value(40% over). The above findings suggest the operation of a protective mechanism, which causes hypoxic pulmonary vasoconstriction in the unilateral hypoxic lung, and diverts blood flow from the hypoxic to the non-hypoxic lung and so minimizes the hypoxic effect on the arterial blood.