The effects of acetone and isopropanol on the microsomal monooxygenase system have been investigated to study the role of this enzyme system in the metabolism of nitrosamines. Treatment of rats with acetone or isopropanol (2.5-5 ml/kg, i.g.) causes a 3-4.5-fold enhancement in the NADPH-dependent nitrosodimethylamine demethylase (NDMAd) activity. This is accompanied by only moderate increases in the gross cytochrome P-450 (P-450) content and NADPH-cytochrome c reductase activity. Several other monooxygenase activities were increased to different extents from an 8% increase in aryl hydrocarbon hydroxylase to a 261% increase in ethoxycoumarin O-dealkylase activities. Kinetic analysis indicates that a low Km form of NDMAd (Km = 0.07 mM) is induced by these treatments. In the microsomes of the treated rats, this high affinity form becomes predominant, in contrast to control microsomes which possess at least three Km-values for NDMAd. The treatment also enhances the metabolism of nitrosomethylethylamine, nitrosomethylbenzylamine and nitrosomethylaniline although to lesser extents than with nitrosodimethylamine. Several lines of observations suggest that the enhanced NDMAd is due to the induction of one or more specific P-450 isozyme(s) by pretreatment with acetone or isopropanol: (a) The treatment induces proteins with molecular weights (Mr) of 50 000 and 52 000 which are in the range of known P-450 isozymes. (b) The induction of these proteins and NDMAd activity was inhibited by CoCl2 and cycloheximide. (c) The induced microsomes had a peak at 450.6 nm different from the 450.0 nm peak of control microsomes. When added to the incubation mixture, both acetone and isopropanol inhibit NDMAd activity. Isopropanol is more potent than acetone and is shown to be a competitive inhibitor with a Ki-value of 0.151 mM.