Possible mechanisms of increased serum aryl sulfotransferase (AST) isozyme activities in cholestatic rats were studied. Serum AST-I, II and -III, IV isozymes activities were determined from the experimental rats with common bile duct ligation (CBDL) or choledocho-caval shunt (CCS). The activities of serum AST-I, II and -III, IV isozymes were found to be increased significantly in both the CCS plus taurocholic acid (TCA) injected group, and the CBDL plus TCA group than those in each control group, such as CCS or CBDL alone groups. The above results suggest that the elevated serum AST most likely due to increased hepatocyte membrane permeability caused by TCA mediated liver cell necrosis.