Oral administration of a single dose of a-naphthyl-isothiocyanate (ANIT) to rats produced a conjugated hyperbilirubinaemia, maximal at 2 days and which subsided by 7. The activities of 3 liver plasma membrane enzymes, Mgz+-ATPase, (Na+-K+)-ATPase and 5’-nucleotidase, and serum bilirubin levels were studied for up to 7 days after treatment. Activities of the 3 enzymes were significantly decreased at 2 days after treatment and returned to normal by 7, thus varying inversely with the degree of hyperbilirubinaemia. Enzyme histochemistry used to demonstrate canalicular localization of Mg2+-ATPase in sections of whole liver and of isolated plasma membrane pellets showed that the reduction in activity was not a uniform partial loss, but represented a range of reductions in most canaliculi with a few retaining normal staining intensity. The results suggest that after ANIT intoxication there is a membrane lesion which may be responsible for the observed hyperbilirubinaemia due to the failure of secretion of biliary constituents into the canaliculus. However, more direct studies are necessary to determine whether any one of these enzymes is directly involved in the transport of biliary constituents across the bile canalicular membrane.