The effect of oral administration of praziquantel at different dose levels on the activities of metabolizing hepatic enzymes (aminopyrine N-demethylase, aniline 4-hydroxylase and UDP-glucuronyltransferase) was studied in healthy locally bred rabbits. The pathological changes resulting from drug's toxicity were assessed histologically, by measurement of total plasma protein concentration and of the activities of the plasma enzymes sorbitol dehydrogenase (SD), glutamate dehydrogenase (GD) and aspartate aminotransferase (AST). No significant changes were obtained after praziquantel administration at dose levels of 40 and 800 mg/kg body weight, whereas 1600 mg/kg and 2000 mg/kg of praziquantel resulted in a significant decrease in the activities of the three drug-metabolizing hepatic enzymes under investigation in the livers of treated rabbits. All rabbits which received praziquantel at the dose rate of 2000 mg/kg died 10-20 hours following praziquantel treatment.