The effects of subclinical fasciolosis at various stages of its development on bile flow and bile acid secretion and on the hepatobiliary transport of bilirubin were investigated in experimentally infected sheep. Bile flow was significantly reduced by weeks 6-14 postinfection. This was accompanied by a decrease in bile acid secretion by weeks 6-8. Serum AST and GLDH activities and serum bile acid concentration were significantly elevated from weeks 6 to 14. Total serum bilirubin was maximally increased at 6 weeks postinfection and remained elevated at weeks 8 and 14. Increases corresponded to both unconjugated and conjugated fractions, although the conjugated/total bilirubin ratio was enhanced in all infected animals. Biliary bilirubin secretion declined from weeks 6 to 14. No alteration was detected in either uridine diphosphate (UDP)-glucuronosyltransferase activity, cytochrome P-450 concentration, or hematological markers of hemolysis. This study shows that the migration of immature flukes in the course of ovine fasciolosis causes a cholestatic phenomenon responsible for changes in serum and biliary bilirubin levels.