Butylated hydroxytoluene (BHT) containing the stable isotope 13C was synthesized from 2-[13C]methylpropan-2-ol. A minor constituent of urine following ingestion of BHT-13C by a human volunteer was identified as 3,5-di-(1-[13C]methyl-1-methylethyl)-4-hydroxybenzoic acid, The major metabolite detected was 13C-labeled 5-carboxy-7-(1-carboxy-1-methylethyl)-3,3-dimethyl-2-hydroxy-2,3-dihydrobenzofuran. Detailed spectral analysis provided the basis for structural assignment. No evidence was found for intermediate metabolites leading from BHT-13C to the above compounds. Analysis of one fraction indicated the presence of traces of S-(3,5-di-(1-[13C]methyl-1-methylethyl-4-hydroxy-benzyl)-N-acetylcysteine. Analysis of serum samples by gas chromatography detected only trace quantities of unchanged BHT. No evidence was found for the presence of other nonpolar or unconjugated metabolites.