Data from double-blind, placebo-controlled trials of the monoamine oxidase (MAO) inhibitors show that phenelzine is clearly effective in neurotic or atypical depressives, but the findings concerning its effect in endogenous depressives are inconclusive. Although few controlled studies have been done with tranylcypromine, similar conclusions are warranted. Studies have contrasted MAO inhibitors and tricyclic antidepressants (TCAs) to gain further information about the type of patients likely to respond to MAO inhibitors. We believe that simply contrasting the relative efficacy of TCAs and MAO inhibitors is outdated. Neurotic or atypical depression is probably a heterogeneous syndrome, and delineation of subtypes responsive to specific antidepressants is needed. The implications of fast acetylation, selective MAO inhibitors, types MAOA and MAOB, and measures of platelet MAO inhibition are discussed in this article.