beta Phenylethylamine (PEA) is an endogenous amine that resembles amphetamine in chemical structure and has been identified in human and other mammalian brains and in many peripheral tissues. Although PEA exists in much smaller amounts than monoamines, the mode of action of PEA as a neuromodulator is of special interest. In this paper the author reviews animal and clinical studies on PEA. The first section deals the methods for the determination of PEA in mammalian brains and human urine. PEA is reliably measured using spectrofluorometric, gas chromatographic-mass spectrometric methods. The second chapter reviews the changes in animal behavior elicited by PEA. The administration of PEA induces stereotypy and increased motor activity in animals. The third part reviews the effect of PEA on dopamine, noradrenaline, serotonin and tyramine metabolism in mammalian brains. The fourth chapter examines recent clinical evidence suggesting a role for PEA in schizophrenia and affective disorders.