Hemodynamics, hemorrheology and blood gases were studied in 20 patients with stable chronic cor pulmonale. Blood viscosity was affected by hematocrit and blood gases, which acted on pulmonary pressure, pulmonary vascular resistance and right cardiac output. Blood viscosity was measured in the shear stress rate of 5.75, 11.5, 23, 46, 115 and 230 s-1. The decrease in plasma pH and PaO2 elevated high shear stress blood viscosity, pulmonary arterial pressure, total pulmonary vascular resistance and decreased right cardiac output, O2 administration decreased in high shear stress blood viscosity, but increased in low shear stress blood viscosity. Blood viscosity was positively correlated with hematocrit, which varied in different shear stress rate, the relative coefficient was the highest when the shear stress rate was in 11.5 s-1 (r = 0.9288 P less than 0.001). Pulmonary arterial pressure and pulmonary vascular resistance were positively correlated with high shear stress blood viscosity but uneffected by low shear stress blood viscosity. The increasing in high shear stress blood viscosity decreased the right cardiac output. Rigidity of red blood cell may play an important role in regulating pulmonary circulation and right heart function.