In brain capillary endothelium and catecholaminergic terminals a single decarboxylation step effected by aromatic amino-acid decarboxylase converts phenylalanine to phenylethylamine, at a rate comparable to that of the central synthesis of dopamine. Phenylethylamine, however, is not stored in intra-neuronal vesicles and is rapidly degraded by monoamine oxidase-B. Despite its short half-life, phenylethylamine attracts attention as an endogenous amphetamine since it can potentiate catecholaminergic neurotransmission and induce striatal hyperreactivity. Subnormal phenylethylamine levels have been linked to disorders such as attention deficit and depression; the use of selegiline (Deprenyl) in Parkinson's disease may conceivably favour recovery from deficient dopaminergic neurotransmission by a monoamine oxidase-B inhibitory action that increases central phenylethylamine. Excess phenylethylamine has been invoked particularly in paranoid schizophrenia, in which it is thought to act as an endogenous amphetamine and, therefore, would be antagonized by neuroleptics. The importance of phenylethylamine in mental disorders is far from fully elucidated but the evolution of phenylethylamine concentrations in relation to symptoms remains a worthwhile investigation for individual psychotic patients.