Incubations of 1-benzylpyrrolidine (4) and specifically deuterium-labeled analogues of 4 with rabbit liver microsomal preparations in the presence of cyanide ion have led to the characterization of 1-benzyl-2-cyanopyrrolidine (13), cis- and trans-1-benzyl-2,5-dicyanopyrrolidine (14a and 14b, respectively), and 1-benzyl-5-cyano-2-pyrrolidinone (15). The cyano adducts of the amine are thought to result from nucleophilic attack by cyanide ion on metabolically generated iminium species. The cyanolactam may be produced by mixed function oxidation of the dicyano compounds. Incubations of tritium-labeled 1-benzylpyrrolidine with rabbit liver microsomal preparations led to the reduced nicotinamide adenine dinucleotide phosphate dependent incorporation of the label into the macromolecular fraction isolated from the postincubates. Although the level of incorporation was low compared to the amount of cyano adducts formed, it is comparable to that reported for other metabolically activated cytotoxic agents. Attempts to identify the possible arene oxide rearrangement product 1-(4-hydroxybenzyl)pyrrolidine (24) as a metabolite of 4 were unsuccessful. The results have prompted us to postulate that metabolically generated iminium ions are capable of alkylating nucleophilic functionalities present on microsomal macromolecules.