In order to elucidate which is the major functional unit, tubulus or glomerulus, on bilirubin excretion into urine, toad (Rana Clamitans) was used for the experiment because the distribution of blood vessels in toads was anatomically well separated to glomerulus and tubulus. (3)H-bilirubin was prepared with Wilzbach's method. Crude direct (3)H-bilirubin was extracted from the rat bile after injection of (3)H-bilirubin dissolved in rat serum into the duodenal canal. Crude direct (3)H-bilirubin were fractionated into three bilirubin fractions named indirect, salt-form and ester-form bilirubin by Kosaka-Hara's method. Change of excretion rate of ester-form bilirubin into urine in various blood pH ranged from 6.4 to 7.4 was investigated from the point of bilirubin-protein binding. The results were as follows; 1. (3)H-radio activity in the urine was bound with the indirect bilirubin at the rate of 85.78% in average and with the ester-form bilirubin at the rate of 93.81% in average. 2. Ester-form bilirubin was excreted into the urine 1.69% in average by loading it to the renal portal vein, in the other hand, that of indirect bilirubin was 0.57% in average, in the first 5 minutes after loading. 3. Loading bilirubins to the renal artery, ester-form bilirubin was excreted into the urine, 10.60%, in average, indirect bilirubin was 0.08%, in average. 4. Excretion of ester-form bilirubin increased according to lowering blood pH, and reached maximum at pH 6.8. 5. As the results, most parts of bilirubin excreted into the urine was the ester-form bilirubin. The site of excretion of bilirubin was mainly tubulus and its rate of excretion might depend on affinity between serum protein and bilirubin.