Data from first-order galactose clearance were used to estimate "effective" hepatic blood flow in normal subjects and in patients with hepatic diseases. Galactose clearance was determined by infusing galactose intravenously at a constant rate and measuring its resulting steady-state concentration in blood. The infusion rate must not exceed the maximum rate of galactose clearance by the liver, the "galactose elimination capacity." With this constraint and within the physiological and pathophysiological limits of hepatic blood flow, a constant infusion at 50 mg/min results in steady-state concentrations ranging from 20 to 200 mg/L. To measure galactose within this range, we modified a YSI Model 23A glucose analyzer and, using an immobilized galactose oxidase (EC 1.1.3.9)/hydrogen peroxide electrode system, accurately measured galactose in water and blood. The galactose metabolic clearance rate was calculated for six normal rats and for a normal and a cirrhotic human subject. The speed of the analysis (40 s), the small sample required (25 microL), and the suitability of fresh whole blood as the sample make the method ideal for measuring hepatic blood flow in small laboratory animals as well as for determining at bedside the effective hepatic blood flow in humans.