The inactivation of cytochrome P450 enzymes by cyclopropylamines has been attributed to a mechanism involving initial one-electron oxidation at nitrogen followed by scission of the cyclopropane ring leading to covalent modification of the enzyme. Herein, we report that in liver microsomes N-cyclopropylbenzylamine (1) and related compounds inactivate P450 to a large extent via formation of metabolic intermediate complexes (MICs) in which a nitroso metabolite coordinates tightly to the heme iron, thereby preventing turnover. MIC formation from 1 does not occur in reconstituted P450 systems with CYP2B1/2, 2C11 or 2E1, or in microsomes exposed to gentle heating to inactivate the flavin-containing monooxygenase (FMO). In contrast, N-hydroxy-N-cyclopropylbenzylamine (3) and N-benzylhydroxylamine (4) generate MICs much faster than 1 in both reconstituted and microsomal systems. MIC formation from nitrone 5 (PhCH = N(O)cPr) is somewhat faster than from 1, but very much faster than the hydrolysis of 5 to a primary hydroxylamine. Thus the major overall route from 1 to a P450 MIC complex would appear to involve FMO oxidation to 3, further oxidation by P450 and/or FMO to nitrone 5' (C2H4C = N(O)CH2Ph), hydrolysis to 4, and P450 oxidation to alpha-nitrosotoluene as the precursor to oxime 2 and the major MIC from 1.