The effects of halothane (H) or enflurane (E) concentration on the circulatory system were studied in dogs. Two hours of halothane or enflurane anesthesia resulted in a linear dose-dependent decrease in circulatory indices inculding: mean arterial pressure (mAP), heart rate (HR), cardiac index (CI) and left ventricular peak dp/dt/IP (peak dp/dt/IP). Systemic vascular resistance (SVR) did not change during either anesthesias. The correlations between the percent change of circulatory indices and the logarithm of the blood anesthetic concentrations were expressed by correlation coefficients (r): mAP, r=-0.718 (H), and -0.650 (E); HR, r=-0.329 (H), and -0.352 (E); CI, =-0.597 (H), and -0.596 (E); SVR, r=-0.161 (H), and -0.030 (E); peak bp/dt/IP, r=-0.708 (H), and -0.871 (E). Using several indices of anesthetic depth including MAC, MAC-EI and MAC-BAR, the percent change of mAP, CI and peak dp/dt/IP were calculated at the same anesthetic depth using halothane or enflurane. These results indicated that enflurane depressed these circulatory indices more than halothane. These defferences were: mAP, 14.51±1.46%; CI, 8.14±1.86%; peak dp/dt/IP, 7.38±3.95% (mean±SD). These results, indicate that the depression of circulatory indices at the same anesthetic depth could be expressed by numerical values.