Phenylethylamine was extracted into n-hexane from alkaline urine saturated with sodium chloride, and back-extracted into dilute acid. The extract was freeze-dried and the residue converted to a pentafluorobenzoyl derivative for analysis by gas chromatography on a column of OV-225 with electron-capture detection. Quantification was achieved by adding an internal standard of tolylethylamine to each sample prior to extraction. Output values in normal subjects and in some patients with phenylketonuria and hyperphenylalaninaemia were in agreement with those in some other recent reports.