The volume of bile produced by isolated perfused rat liver was dependent on the concentration of bovine serum albumin (BSA) in the perfusate, the lowest volumes being obtained with Krebs Ringer bicarbonate (KRB alone and with 70 g/l. BSA in KRB. Increasing amounts were produced as the concentration of BSA was reduced from 70 g/l. to 8.5 g/l. The concentration of bile salts in the bile decreased rapidly during the first hour, but much more slowly thereafter. The highest concentration observed during the first hour was with KRB alone and the lowest with 20 g/l. BSA; subsequently, the highest concentrations were obtained with 70 g/l. BSA and the lowest with 20 g/l. BSA. Urea synthesis was lowest with KRB alone, but increased as the concentration of BSA increased; however, values obtained with 8.5, 20, and 40 g/l. were fairly close together. With KRB alone there was a rapid release of potassium during the first hour, but thereafter the rate of release was similar to that found when BSA was present. The lowest concentrations of potassium were obtained with 20 and 40 g/l. BSA. With KRB alone, GOT was released from the start of perfusion of 300 units per minute. With BSA present, GOT release was delayed 4 hours, and thereafter the rate was dependent on the concentration of BSA, being lowest with 20 and 40 g/l. (200 units per min), higher with 8.5 g/l. (350 units per min) and highest with 70 g/l. (400 units per min).