The purpose of this study was to estimate hepatic blood flow during halothane (HAL) or isoflurane (ISO) anaesthesia with the noninvasive indocyanine green (ICG) clearance test. Twenty-four ASA status I adult patients, scheduled for elective surgery without liver disease, were allocated into four groups. Before surgery, ICG clearance was measured twice in patients before and after 1, 2 MAC HAL-N2O, 1 and 2 MAC ISO-N2O anaesthesia by ICG clearance meter (Sumitomo Electronics, Japan). This method eliminates the blood sampling and delay of the conventional ICG test. The ICG clearance is displayed in two indices: K (ICG disappearance rate) and R15 (ICG retention rate 15 min after 0.5 mg.kg-1 ICG injection). Indirect blood pressure and heart rate were measured simultaneously. Mean arterial pressure (MAP) decreased by 27, 22 and 29% with 2 MAC HAL-N2O, and with 1 and 2 MAC ISO-N2O groups, respectively (P < 0.05). The ICG clearance was less (P < 0.05) in the 2 MAC HAL-N2O group (K, 0.087 min-1) than with either awake (K, 0.131 min-1) or other groups (K, 0.114 and 0.144 min-1, in the 1 MAC HAL and ISO-N2O groups, respectively) in spite of a similar degree of the depletion of MAP in 2 MAC ISO-N2O group (K, 0.124 min-1). We conclude that isoflurane has a more favourable effect on liver circulation than does halothane.