The phenethylamine (PEA) level in daytime urine samples of 18 normal subjects was measured by an isotope dilution technique. Purification of PEA from other urinary substance, after the addition of 1-14C-PEA to urine samples, was achieved by the sequence: thin-layer chromatography (TLC), derivatization, TLC, and finally, gas chromatography (electron capture). The estimated mean daily output of PEA was 10.3 mug/day. A circadian rhythm was not evident on the basis of analyses of 24-hr urine collections from four normal subjects. Three acute schizophrenics had levels within the normal means.