1. The metabolism of cyclopropylbenzene (1a) and 4-cyclopropylanisole (1b) was studied using liver microsomal preparations from control, phenobarbital- andβ-naphthoflavone treated rats.2. With all three types of microsomes1awas metabolized by benzylic hydroxylation to give 1-phenylcyclopropanol and by aromatic hydroxylation at C-4; the former predominated by a factor of 2–4. BNF-induced microsomes also formed 2-cyclopropylphenol. No cyclopropyl ring-opened metabolites of1a, including benzoic acid, were detected in any of the incubations.3. With PB-induced microsomes1bunderwentO-demethylation (90%) and benzylic hydroxylation; no other metabolites were detected.4. Progress curves for metabolism of1aare markedly nonlinear after only limited conversion of substrate, suggesting the possibility that1a, like other cyclopropyl compounds, could be a suicide substrate for one or more isozymes of P450.5. For both1aandb, metabolite formation and enzyme inactivation can be explained by conventional P450 reaction mechanisms not involving electron abstraction.