1. After administration to rats of methyl acrylate (I), methyl methacrylate (II) and methyl crotonate (III), urinary mercapturic acids were isolated and identified as the dicarboxylic acidsN-acetyl-S-(2-carboxyethyl)cysteine (IV, R = H),N-acetyl-S-(2-carboxypropyl)cysteine (V, R = H) andN-acetyl-S-(1-methyl-2-carboxyethyl)cysteine (VI, R = H) and for a minor part as their monomethyl esters IV (R = CH3), V (R = CH3) and VI (R = CH3).2. After a single dose of the acrylates (I), (II) and (III) (0·14mmol/kg), the excretion of the thioethers amounted to 6·6±0·6, 0·0, and 2·0±0·6% dose respectively.3. After 18 h previous administration of the carboxylesterase inhibitor tri-o-tolyl phosphate (0·34mmol/kg) the excretion of the thioethers amounted to 40·6±2·1, 11·0±3·3, and 16·0±2·0% dose.4. For methyl acrylate (I) the ratio of the excreted dicarboxylic acid and monomethyl ester was 20:1. After previous administration of tri-o-tolyl phosphate this ratio was 1 : 2.