Canalicular membrane alteration was investigated as a possible mechanism for the cholestasis induced by a manganese-bilirubin (Mn-BR) combination. Hepatic bile canalicular (BCM)- and sinu-soidal (PM)-enriched membrane fractions were isolated under both cholestatic (Mn-BR or bile duct ligation) and noncholestatic (manganese or bilirubin alone; manganese + sulfobromo-phthalein + bilirubin) conditions. the major effect of Mn-BR interaction was an important shift in the recovery of proteins in isolated liver plasma membrane fractions; the shift was from the canalicular to the sinusoidal fraction. Enzymic activities (5'-nucleotidase, leucylaminopeptidase and ATPase) ofboth BCM and PM fractions were easily depressed even if bile flow was normal and therefore do not appear to be good indicators of altered bile secretory function. Canalicular membranes isolated from animais in Mn-BR cholestasis were yellow and probably incorporated bilirubin. Phalloidin pretreatment afforded progressive and reversible protection against Mn-BR-induced cholestasis.