Biliary flow and the biliary excretion of bromcresol green were measured in rats injected i.p. with a single dose of phenobarbital, 75 mg/kg. Biliary flow began to increase 6 h after the injection, it reached a peak at 36 h and returned to the control level at 72 h. In the same rats, the enhanced biliary excretion of bromcresol green was first observed at 12 h, it reached a peak at 24 h and returned to the control level at 72 h. Other groups of rats received 75 mg/kg phenobarbital as daily i.p. dose over 5 days. In these groups, the increase in biliary flow did not exceed that measured in the rats given the single dose, however, the biliary excretion of bromcresol green reached its peak after a 4-day phenobarbital treatment. After the 5th day of treatment the biliary flow and the biliary excretion of bromcresol green remained significantly stimulated for 4 days and the changes in these parameters regressed on the fifth day following the last injection of phenobarbital. The changes in liver weight appeared not to run closely parallel either with the increase in biliary flow or with the enhancement of biliary excretion of bromcresol green. It is suggested that the changes in biliary flow and in the biliary excretion of bromcresol green take place by different mechanisms after phenobarbital administration.