The present study was designed to determine changes in hepatic oxygen supply in guinea pigs during halothane or isoflurane anesthesia. Twenty-seven guinea pigs were randomly divided into three equal groups: control (no anesthesia) group, and animals anesthetized with halothane or isoflurane to decrease mean arterial pressure (MAP) by 50%. Hepatic arterial blood flow (HABF) and portal blood flow (PBF), as well as arterial and portal venous blood oxygen content, were determined in awake animals (stage I, baseline values), and during anesthesia (stage II). HABF was found to be extremely low (0.04 ml.min-1.g-1) during both stages of observation in the control (no anesthesia) group, as well as during stage I (awake) in animals treated with halothane or isoflurane. Equal degrees of arterial hypotension during halothane and isoflurane anesthesia were accompanied by decreased HABF during halothane (37%), but no significant change in HABF during isoflurane anesthesia. PBF decreased significantly in both experimental groups; however, the decrease was more prominent during halothane than during isoflurane anesthesia (57% vs. 23%). The observed hepatic circulatory changes led to a 65% decrease in hepatic oxygen delivery during halothane, but only a 34% decrease during isoflurane anesthesia. The present study does not exclude the possibility that liver damage in the guinea pig model is related to the reductive metabolism of halothane or any other mechanism. However, the extremely low HABF and a prominent reduction in both HABF and PBF during halothane anesthesia may be responsible for hepatic damage observed in the guinea pig model.