1. The kinetics of the plasma disappearance of bilirubin (2 mg/kg intravenously) were studied in 106 patients with Gilbert's syndrome and in 13 normal subjects. 2. All patients had significant decreases in hepatic bilirubin clearance and transfer rates from plasma to liver, resulting in increased values for plasma retention at 4 h. The calculated value for unconjugated bilirubin production was normal in 40% of patients and increased in the remainder. 3. In 29 of the Gilbert's patients their bromosulphthalein kinetics were studied 1 week before the bilirubin test. These results were essentially normal and it was concluded that the hepatic clearance mechanisms for bilirubin and bromosulphthalein are different. 4. In 10 patients the bilirubin transport maximum ( T m ) was found to be low whereas the relative storage capacity ( S ) was normal. Phenobarbitone treatment in four patients resulted in an increase in T m , and S decreased in two patients and remained unchanged in the other two. 5. These results support the hypothesis that there are several variants of Gilbert's syndrome and that the bilirubin tolerance test is a useful diagnostic test.