Phenelzine-d5 (sulfate) is intended for use as an internal standard for the quantification of phenelzine by GC- or LC-MS. Phenelzine is an inhibitor of monoamine oxidase (MAO; IC50 = 0.9 μM using rat brain mitochondrial preparations).1 It potentiates the effects of tryptamine on isolated rat fundus (EC50 = 90 nM) and increases tryptamine toxicity in mice with LD50 values of 85 and 500 mg/kg in the presence and absence of phenelzine, respectively. Phenelzine (20 mg/kg) increases GABA, dopamine, serotonin , and norepinephrine levels in the hippocampus and cortex of socially isolated rats and rats treated with the NMDA receptor antagonist (+)-MK-801 .2 It also increases 5-HT levels in the ventral horn of the spinal cord, improves gross motor ability in a rotarod test, and increases locomotor activity in an open field test in mice with experimental autoimmune encephalomyelitis when administered at a dose of 30 mg/kg.3