The evidence implicating the formation of iminium ions as reactive intermediates in the metabolism of alicyclic amines has been reviewed. The mechanism of formation of iminium ions and their conversion to alpha-carbonyl compounds or demethylated amines is discussed. The use of a simple cyanide trapping technique for iminium ions has been demonstrated to monitor a large number of alicyclic drugs for iminium ion formation. The possible role of iminium ions in the pharmacology and toxicology of alicyclic amines is considered.