The purpose of the study was to evaluate (on the basis of pharmacokinetics of antipyrine) the hepatic biotransformation rate in calves in neonatal period. The calves received supplementary casein or glucose in their food ration. It was found that an increased protein content in the diet significantly increased the hepatic biotransformation rate as indirectly indicated by the monooxygenase activity linked to cytochrome P450. Increased carbohydrate content in the diet produced an opposite effect upon the activity of the system. From these results it can also be concluded that a high-protein diet would increase whereas a high-carbohydrate diet would decrease the metabolism of other of drugs in calves, provided they would undergo the same biotransformation process as antipyrine does. AllIipyrine. neonatal period. casein. glucose. liver. MFO system