A review of the literature indicates that brain phenylethylamine (PEA) may be a neuromodulator of aminergic synapses and that it promotes energy, elevates mood, and favors aggression. Phenylacetic acid, the main metabolite of PEA, is decreased in the biological fluids of depressed subjects and schizophrenic subjects and is increased in schizoaffective subjects. The administration of PEA or of its precursor L-phenylalanine improves mood in depressed patients treated with a selective monoamine oxidase B inhibitor. The authors speculate that studies of PEA metabolism may have diagnostic value and that PEA administration may be therapeutic in selected depressed patients.