The immediate evolution of changes in serum bilirubin and alkaline phosphatase levels during and after brief temporary occlusion (12 hours or less) of the main bile duct was studied in the rat. Transient obstruction of the main bile duct of more than one hour was characterized by a simultaneous increase in serum levels of both bilirubin and alkaline phosphatase. The time required for return to normal values was less for bilirubin than the duration of a brief obstruction and was more prolonged for alkaline phosphatase than the duration of obstruction. In obstruction of four or more hours, it was rare to find an isolated elevation of serum bilirubin; this occurred when the control value for alkaline phosphatase levels was low and their relative increase was obscured. Thus, an isolated increase in the serum bilirubin level is infrequently compatible with obstruction of the bile duct, even when transient and of brief duration. Conversely, isolated elevation of serum alkaline phosphatase levels in an acute abdominal syndrome may indicate relieved biliary obstruction.