Male Wistar rats were given pyrazole subcutaneously at a dose of 100 mg/ kg daily, for 4 days, and the in vitro activities of drug metabolizing enzymes and the concentrations of copper, zinc and magnesium were measured in the liver. During the course of treatment, the animals failed to thrive at the rate of the controls. Although protein content of microsomes was unchanged, the phospholipid content increased. Pyrazole administration significantly decreased the activity of NADPH-cytochrome c reductase, while it did not affect the O-demethylation of p-nitroanisole and the microsomal content of cytochrome P-450. The activities of UDPglucose dehydrogenase, UDPglucuronosyltransferase and l-gulonate dehydrogenase were markedly enhanced. On the contrary, pyrazole administration produced a decrease in the activities of UDPglucuronic acid pyrophosphatase, β-glucuronidase and d-glucuronolactone dehydrogenase. The pentobarbital sleeping-time was prolonged after pyrazole treatment for 4 days but the animals showed a decreased spontaneous locomotor activity already on the third day of treatment. This suggests that a depression of the central nervous system was present, which was due to pyrazole itself. The total hepatic content of copper was significantly higher in pyrazole-treated animals than in the control group. The concentrations of zinc and magnesium were increased in the hepatic cytosolic fraction. These results indicate that pyrazole affects the enzymes involved both in hydroxylation and in glucuronidation reactions. In view of the unspecificity of the compound in affecting various enzyme systems, its use in studies on ethanol metabolism in vivo does not seem warranted.