The metabolism of methyl, ethyl, butyl and allyl isothiocyanate, which occur as glucosinolates in a number of plants, was studied. Oral administration of the substances to the rat was followed by their renal excretion as mercapturic acids, which were isolated as dicyclohexylamine salts. Chemical structure was determined by synthesis and 1H-n.m.r. spectra. The mercapturic acids were very labile dithiocarbamidic acid esters, formed by the addition of the isothiocyanate group to the SH group of the cysteine component.