The cause of postoperative liver dysfunction is often unclear, but a decrease in liver blood flow during anesthesia and/or major surgery may be important. Plasma half-life and clearance of indocyanine green were therefore measured in 42 patients before, during and after anesthesia and abdominal surgery. In 13 patients, liver blood flow was also estimated from indocyanine green extraction using hepatic vein catheterization. The major finding was an early decrease in estimated liver blood flow after induction of anesthesia, but not later during or after surgery. Mean indocyanine green half-life increased by 26% (p < 0.005), mean indocyanine green plasma clearance decreased by 19% (p < 0.01) and mean estimated hepatic blood flow decreased by 36% (p < 0.005) during the first 30 min of anesthesia. These changes were greater in males and in patients older than 55 yr, but changes were similar with each of three anesthetic groups. Half-life, but not indocyanine green clearance, was also significantly prolonged by mid-operation in the older (>55 yr) patients and in those undergoing lower abdominal surgery. We suggest that this period of reduced liver blood flow during anesthesia is caused by the effects of neuromuscular blocking agents and may contribute to postoperative liver damage. (HEPATOLOGY 1991;14:1161–1166.)