Pyrazole, a potent inhibitor of alcohol dehydrogenase, was found to be a potent inducer of the activity of low Km dimethylnitrosamine demethylase (DMN-d). One injection of pyrazole (200 mg/kg body wt) to weanling Wistar rats changed the microsomal DMN demethylase activity by 1.7, 1.9 and 2.5 times the control values at 6, 12 and 24 hr after the injection respectively. Pyrazole administration reduced arylhydrocarbon hydroxylase (AHH) activity. When animals were injected with pyrazole (200 mg/kg body wt) for 1, 2, 3 or 4 consecutive days, the values for DMN-d activity were 277, 297, 306 and 319% of the control values. The corresponding values for AHH were 91, 67, 57 and 45% for 1, 2, 3 and 4 injections respectively. pyrazole-induced DMN-d activity was NADPH dependent and was inhibited by CO; n-butanol gave a 50% inhibition at a concentration of 2 X 10(-3) M. The corresponding value for metyrapone was 1 X 10(-2) M. Cytochrome P-450 was slightly increased by pyrazole and its CO-complex gave an absorption maximum around 451 nm. When the microsomal proteins were separated using sodium dodecylsulfate (SDS)-polyacrylamide gel electrophoresis, a large increase in a band at about 51,000 daltons was found in the liver microsomes of pyrazole-treated animals.