Low-dose galactose clearance is a new method for measuring functional (nutrient) liver blood flow. In 22 healthy beagle dogs, the mean (+/- SD) blood galactose clearance rate of 311 +/- 93 mL/min was not significantly different from the mean measurement obtained using electromagnetic flow probes (322 +/- 37 mL/min). This shows that galactose clearance can be used to measure liver blood flow in healthy dogs. The 22 dogs were divided into two groups of 11. The first group underwent portacaval shunting and weekly galactose clearance rates were measured until death an average of 6 weeks later. The anticipated fall in liver blood flow was successfully detected by the second week after shunting. This suggests that long-term (week-to-week) changes in liver blood flow can be detected by this method. In the second group, ligation of the common bile duct was used to induce secondary biliary cirrhosis. Galactose clearance was measured weekly for 6 weeks and showed a significant decrease by 6 weeks. At 7 weeks, laparotomy was performed in order to take flow-probe measurements; the galactose clearance rate was also measured. Whereas the two methods were similar at the time of the original operation, 7 weeks after ligation there was a significant difference (p = 0.02) with the rate of liver blood flow as measured by galactose clearance being much lower than the flow rate measured by the electromagnetic flow probes. These findings suggest that in cirrhotic dogs, galactose clearance measures functional or effective (nutrient) liver blood flow whereas the electromagnetic flow probe measures anatomic flow. The effective flow gives a more accurate reflection of perfusion of the hepatocyte by blood.