1. The clearance of a tracer dose of [14C]bilirubin from the plasma was studied in patients with Gilbert's syndrome, congenital non-haemolytic jaundice, haemolytic jaundice and in normal subjects. Clearance was significantly impaired in the patients with Gilbert's syndrome and in those with congenital non-haemolytic jaundice when compared with the normal subjects, and was normal in the patient with haemolytic jaundice.

2. Treatment for 2 weeks with phenobarbitone (180 mg/day) lowered the plasma bilirubin concentration and improved all indices of plasma clearance of the isotope in patients with Gilbert's syndrome, so that they became indistinguishable from those of normal subjects. The improvement in plasma [14C]bilirubin clearance in these patients was associated with modest increases in hepatic bilirubin glucuronyl transferase in some subjects.

3. Phenobarbitone treatment improved plasma bilirubin concentrations and plasma [14C]bilirubin clearance in patients with congenital non-haemolytic jaundice, so that they resembled those seen in patients with untreated Gilbert's syndrome. Despite this improvement hepatic bilirubin glucuronyl transferase activity remained undetectable.

4. These results are compatible with the hypothesis that Gilbert's syndrome is a manifestation of a relative deficiency of hepatic bilirubin glucuronyl transferase, and differs from congenital non-haemolytic jaundice only in severity.

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