The therapeutic effects of intentional hemodilution (IHD) on 64 patients of rheumatic mitral valvular disease with chronic pulmonary stasis were estimated. The mean hematocrit was decreased from 0.50 +/- 0.06 to 0.41 +/- 0.04 by withdrawing blood and replacing it with diluent. The whole blood ratio viscosity was decreased from 11.7 +/- 2.4 to 6.4 +/- 0.8 (P < 0.01), the pulmonary arterial pressure was reduced from 6.4 +/- 0.6 to 4.1 +/- 0.5 kPa (0.133 kPa = 1 mmHg) (P < 0.01), the pulmonary vascular resistance brought down from 536.0 +/- 108.0 to 315.0 +/- 62.0 dynes.s.cm-5 (P < 0.01), the cardiac output increased from 3.4 +/- 0.4 to 5.6 +/- 0.5 L/min (P < 0.01), the cardiac index increased from 2.2 +/- 0.5 to 3.5 +/- 0.4 L/min/m2 (P < 0.01), and venous oxygen saturation and oxygen content increased from 28.5 +/- 3.4%, 5.1 +/- 0.5% to 56.0 +/- 5.0%, 11.3 +/- 1.2%, respectively (P < 0.01). The patients' symptoms and signs improved significantly. The protective role of IHD in the early stage of lethal pulmonary edema induced by epinephrine in 39 rabbits and 27 rats was also observed. The experimental data show that intentional hemodilution increased the animals' survival rate (P < 0.01), reduced the pulmonary coefficient, permeability index of Evans blue and protein content in broncho-alveolar lavage fluid (BALF) (all P < 0.01), and alleviated the pathomorphological changes of the lungs of animals in Group IHD were alleviated. The results suggest that IHD acts well in both chronic pulmonary stasis and acute pulmonary edema.