Phenelzine, β-phenylethylhydrazine, of structural formula, is regarded as a potent, rapidly acting, long-lasting monoamine oxidase (M.A.O.) inhibitor. The administration of such compounds protects 5-hydroxytryptamine (serotonin) which is destroyed by M.A.O. 5-hydroxytryptamine is believed to act in the brain as a chemical mediator, the function of which is to control the pulsating action of oligodendroglial cells which supply the other brain tissues with nutrient. It has been suggested that a relative 5-hydroxytryptamine deficiency is the fundamental biochemical disorder of severe depressive states and that M.A.O. inhibitors such as phenelzine tend to promote restoration to more normal concentration and activity.