The effects of 4 weeks of treatment with the selective monoamine oxidase (MAO) inhibiting antidepressant clorgyline and pargyline on the sleep of affectively disordered patients were studied. Both inhibitors resulted in near total suppression of REM sleep, a decrease in total sleep time, and an increase in the percent of stage 2 sleep. Clorgyline also increased awake time and decreased total recording period and sleep latency. In general, changes were greater for clorgyline than for pargyline and were about 50% slower to return to baseline after clorgyline compared to pargyline discontinuation. The results were consistent with the hypothesis that selective inhibition of the MAO type A, as produced by clorgyline, is sufficient to induce marked sleep changes. MAO inhibitor-induced receptor changes are proposed to account for the time course of the REM suppression and the REM rebound observed upon withdrawal.