The purpose of this investigation was to determine the effects of experimental dicrocoeliosis on bile formation in the hamster. Studies were carried out at 120 days after infection with an oral dose of 40 metacercariae of Dicrocoelium dendriticum. A significant elevation in bile flow (+20%) and in the biliary output of glutathione (+34%), bile acid (+59%), cholesterol (+108%), phospholipids (+99%) and alkaline phosphatase (+36%) was observed in the infected animals. The bile-to-plasma [14C] mannitol ratio increased to values greater than 1 and there was a reduced contribution (-26%) of biliary tree to bile formation. Those data suggest that enhancement in choleresis had a canalicular origin. The presence of oxidative stress, evidenced by the increased oxidized/reduced glutathione ratio and TBARS concentrations, may contribute to the elevated glutathione efflux into bile. Enhancement in bile acid output was not due to qualitative or quantitative changes in bile acid metabolism, as indicated by the absence of significant modification in liver cholesterol 7alpha-hydroxylase activity and bile acid profile in bile. Increase in the ability of the canalicular membrane to export bile acids was not involved, since maximal secretion rate for exogenously administered taurocholate was decreased. When bile flow, bile acid and biliary lipid secretion was determined in colchicine-pretreated animals differences between control and infected animals were abolished, suggesting that stimulation of the transcytotic vesicle pathway plays an important role in the alteration of the biliary function caused by dicrocoeliosis.