Modaline is piperidinopyrazine derivative developed by Warner-Lambert Pharmaceutical Co. as Antidepressant. Modaline is a potent monoamine oxidase (MAO) inhibitor of the order of the more effective hydrazines. Pretreatment with harmaline, a reversible MAO inhibitor, or with serotonin prevented the irreversible inhibition by Modaline, suggesting that Modaline (as an active MAO inhibitor) occupies the same site on MAO as harmaline, the hydrazines, or serotonin. Modaline differs from the monoamine oxidase inhibitors by producing an increase in body temperature in fully reserpinized animals. Modaline differs from imipramine‐like drugs in not potentiating the hyperthermic effect of isoprenaline.