The effect of bile acids on bile flow and biliary excretion of eosine (80 mg/kg i.v.) has been investigated in anaesthetized (urethane 1.2 g/kg i.p.), bile duct-cannulated female Sprague-Dawley rats weighing 180--220 g. The biliary flow was significantly enhanced by cholic acid, glycocholic acid, taurocholic acid, deoxycholic acid, chenodeoxycholic acid and dehydrocholic acid (each in a dose of 100 mg/kg i.v.). However, it was definitely decreased by lithocholic acid (10o mg/kg i.v.). The biliary excretion of eosine was increased by cholic acid and dehydrocholic acid, whereas it was decreased by chenodeoxycholic acid and lithocholic acid. Deoxycholic acid, glycocholic acid and taurocholic acid had no effect on the biliary excretion rate of eosine. These results suggest that no parallelism exists between the choleretic effect of bile acids and their influence on the biliary excretion of eosin.