Bilirubin load test was performed to differenciate hyperbilirubinemia in various liver diseases. For this purpose, bilirubin excretion pattern, serum direct and indirect bilirubins, and the molar ratio of glucuronic acid to bilirubin in the ester-form bilirubin fraction separated by Kosaka and Hera's method were investigated. Phenobarbital was effective to decrease serum bilirubin in Gilbert's syndrome, so this phenomena was investigated from the point of bilirubin metabolism. The following results were obtained: 1) Serum total bilirubin markedly elevated after bilirubin loading in Gilbert's syndrome, and direct bilirubin was increased significantly, and the molar ratio of glucuronic acid to bilirubin was lower than normal, which was coincided with the low activity of UDPGA bilirubin glucuronyl transferase in liver tissue. 2) Serum total bilirubin after bilirubin loading in Dubin-Johnson syndrome showed high retention, and many peaks were observed in direct bilirubin, which showed high value. Molar ratio flucutiated in normal range. These findings support the prolonged excretion of serum bilirubin in this disease due to the bilirubin circulation from liver cell to microbili. 3) In Rotor syndrome, retention of serum total bilirubin after bilirubin loading was not so high, and serum direct bilirubin increased and the molar ratio was in normal range. 4) Excretion of serum bilirubin slightly delayed in posthepatitic hyperbilirubinemia after bilirubin loading, and serum direct bilirubin and molar raio showed normal pattern. 5) Phenobarbital administration in Gilbert's syndrome was effective to decrease serum bilirubin, and serum bilirubin increased after stopping the drug administration. This phenomen was not only elucidately by the induction of UDPGA bilirubin glucuronyl transfesase activity, but the activation of oxydative and redox state of microsomal enzyme system and the normalzation of liver cell membrane would be participated in the nomalization of serum bilirubin.