The effects of p-tert-butylphenyl trans-4-guanidinomethylcyclohexane carboxylate hydrochloride (NCO-650) and its metabolite, p-tert-butylphenol (BP), on the drug-metabolizing enzymes, lipids and fine structure in rat liver were examined. Male Sprague-Dawley rats, 7 weeks of age, weighing about 200 g were used in the experiment. NCO-650 and BP were given by the gavage at a dose of 2.5, 12.5 and 125 mg/kg, once a day for 5 weeks. The behavioral abnormalities such as inhibition of the gain body weight, piloerection and acceleration of withdrawal reflex were revealed by the treatment of NCO-650 and BP at a dose of 125 mg/kg. The mortality of BP at 125 mg/kg was 20 percent. The induction or inhibition of aminopyrine demethylase, cytochrome b5 and cytochrome P-450 were not continued by the treatment of NCO-650 and BP. The lipid peroxide formation of ascorbic acid-dependent in the mitochondria was increased at 3 weeks after NCO-650 and BP treatment, but that of NA-DPH and ascorbic acid-dependent caused an inhibition in the microsome. The lipid levels in the serum decreased by the treatment of NCO-650 and BP at 125 mg/kg. The levels of total protein, albumin and GOT in the serum decreased by the treatment of BP at 125 mg/kg. The disarrangement of rough endoplasmic reticulum in fine structure of liver were produced at one and 3 weeks after NCO-650 and BP treatment. The toxicity of BP in rat was found to be greater than that of NCO-650. These results suggest that the changes of drug-metabolizing enzymes, lipid peroxide formation and fine structure in the liver were not continued by the subacute treatment of NCO-650 and BP, and indicate that the hepatotoxicity was not produced by these drugs.