Benzylmercapturic acid is a major metabolite of [methylene-14C]benzyl chloride in the marmoset, as in the rat. The excretion of the minor metabolites benzylmercapturic acid sulphoxide and benzylcysteine accounted for a greater proportion of the dose than in the rat. Excretion of hippuric acid as a metabolite of benzyl chloride was variable in the marmoset. Acetylation of S-benzyl- and S-pentyl-L-cysteine to the corresponding mercapturic acids was extensive in the marmoset. Trace amounts of the sulphoxides of these acids were also excreted.