The hypothetisis that monoamine oxidase (MAO) inhibitory activity could account for the clinical effects of the antidepressant drugs, and the subsequent observations that many of the hydrazine drugs proved to be toxic, has focused attention on the nonhydrazine MAO inhibitors. The majority of the compounds examined so far, however, have been closely related to harmahne, i.e. 7-substituted. The ease with which tryptamines, related to the neurohormone serotonin, cyclize to form 6-substituted beta-carbolines has been reported. These compounds, which it has been postulated might arise endogenously, have been found to be potent serotonin antagonists and to affect conditioned behavior. To further elucidate the structure-action relationship of these compounds, a series of beta-carbolines has been assayed as MAO inhibitors