Rapid infusion is believed to be harmful to the lung, however, the pathological status of pulmonary edema resulting from excessive fluid therapy in horses has not been clarified because the quantitative diagnosis of pulmonary edema is impossible. To evaluate the precision of the double indicator dilution method using heat and sodium in horses, which allows the quantitative diagnosis of pulmonary edema, we compared extravascular lung water volume measured using a lung water computer based on the theory of the double indicator dilution method with that determined by the direct method. The value of extravascular thermal volume (ETV) determined by the double indicator dilution method was 7.82 +/- 0.62 ml/kg and the detection ratio of ETV to the value of pulmonary extravascular water volume (PEWV) by the direct method was 0.996 +/- 0.038. There was a significant correlation between ETV and PEWV (P < 0.05), and the regression line was Y = 1.23 X - 1.73 with a correlation coefficient of 0.953. The value of extravascular lung water determined by the double indicator dilution method was significantly consistent with that obtained by the direct method, indicating the high precision of the double indicator dilution method in normal horse lungs.