The reproducibility of the thermodilution method in anesthetized and conscious rats was analyzed. Cardiac outputs in anesthetized state were stable in each procedure. Those in conscious state, especially the first measurement, tended to be underestimated compared with those in anesthetized state. The difference between anesthetized and conscious states might be produced by the increased temperature of saline in the tube placed under the skin of the conscious rat. Therefore, if we exclude the value of the first measurement, the thermodilution method is reliable to measure cardiac output in conscious state.