The suicide substrate activity of N-benzyl-N-cyclopropylamine (1) and N-benzyl-N-(1'-methyl-cyclopropyl)amine (2) toward cytochrome P450 and other enzymes has been explained by a mechanism involving single electron transfer (SET) oxidation, followed by ring-opening of the aminium radical cation (protonated aminyl radical) and reaction with the P450 active site. Although the SET oxidation of N-cyclopropyl-N-methylaniline (3) by horseradish peroxidase leads exclusively to ring-opened (noncyclopropyl) products, P450 oxidation of 3 leads to formation of cyclopropanone hydrate and no ring-opened products, and 3 does not inactivate P450. To help reconcile these discrepant behaviors we have determined the complete metabolic fate of 1 with P450 in vitro. 3-Hydroxypropionaldehyde (3HP), the presumptive "signature metabolite" for SET oxidation of a cyclopropylamine, was observed for the first time in 57% yield, along with cyclopropanone hydrate (34%), cyclopropylamine (9%), benzaldehyde (6%), benzyl alcohol (12%), and benzaldoxime (19%). Unexpectedly, N-benzyl-N-cyclopropyl-N-methylamine (4) was found not to inactivate P450 and not to give rise to 3HP as a metabolite without first undergoing oxidative N-demethylation to 1. These and other observations argue against a role for SET mechanisms in the P450 oxidation of cyclopropylamines. We suggest that a conventional hydrogen abstraction/hydroxyl recombination mechanism (or its equivalent as a one-step "insertion" mechanism) at C-H bonds in 1-4 leads to nonrearranged carbinolamine intermediates and thereby to "ordinary" N-dealkylation products including cyclopropanone hydrate. Alternatively, hydrogen abstraction at the N-H bond of secondary cyclopropylamines 1 gives a neutral aminyl radical which could undergo rapid ring-opening leading either to enzyme inactivation or 3HP formation.