Monoamine oxidase inhibitors (MAOIs) were first identified as effective antidepressants in the late 1950s. An early report suggested that iproniazid, an antitubercular agent, had mood-elevating properties in patients who had been treated for tuberculosis (Bloch et al. 1954). Following these observations, two studies confirmed that iproniazid did indeed have antidepressant properties (Crane 1957; Kline 1958). Zeller (1963) reported that iproniazid caused potent inhibition of monoamine oxidase (MAO) enzymes both in vivo and in vitro in the brain. He also reported that the medication reversed some of the actions of reserpine. Because reserpine produced significant depression as a side effect, it was suggested that iproniazid might have mood-elevating properties.