The metabolism of (±)fenfluramine, 1-( m-trifluoromethylphenyl)-2-N-ethylpropane, an anoretic agent, was investigated in humans. The analysis method was based on the use of ion-exchange resin extraction, solid-phase purification on the Bond Elut™ C 8 cartridge, gradient elution high-performance liquid chromatography, enzymic hydrolysis of conjugates, further purification by Bond Elut C 8 cartridge, derivatisation and capillary column gas chromatography—mass spectrometry (GC—MS). After administration of a 1 mg kg −1 oral dose, four metabolites plus unchanged fenfluramine were recovered in the 0–24 h urine from human volunteers and characterised by GC—MS. In the unconjugated form, fenfluramine, norfenfluramine and m-trifluoromethylhippuric acid were detected by GC—MS. In the aglycone form, the major metabolite, 1-( m-trifluoromethylphenyl)-1,2-propane diol (fenfluramine diol), was monitored using GC—MS. The mass spectral characteristics of the m-trifluoromethylhippuric acid methyl ester, 1-( m-trifluoromethylphenyl)-1,2-propane ditrifluoroacetate derivatives and the norfenfluramine and fenfluramine free base obtained under electron-impact ionization are presented. The metabolism of fenfluramine is discussed including a metabolic pathway in man accounting for the formation of its biotransformation products.