A specific and sensitive radioimmunoassay for the determination of 3,4-dimethoxyphenylethylamine (3,4-DMPEA) in urine was developed. In 5 fasting healthy volunteers excretion of 3,4-DMPEA was not diminished, indicating that this substance can be regarded as an endogenous metabolite. In addition, the time profile of the excretion of 3,4-DMPEA was measured in one patient during total fasting. In 25 patients with schizophrenic psychoses, 9 psychiatric patients without schizophrenia and 80 control patients excretion of 3,4-DMPEA was compared. No significant differences could be observed between these three groups.