Dehydromethionine (S-methylisothiazolidine-3-carboxylic acid) is reduced by glutathione (aqueous solution, 25 degrees C) to give methionine and glutathione disulfide in a reaction that is dependent on pH and the nature and concentration of the buffer utilized. The data are consistent with proton-assisted, rate-limiting attack of glutathione thiolate anion on the sulfilimine sulfur with concomitant cleavage of the sulfur-nitrogen bond. The data predict a half-life for dehydromethionine ranging between 33 and 330 min in the presence of physiological concentrations of glutathione and phosphate buffer, suggesting that dehydromethionine and similar sulfilimines may have a sufficient lifetime in vivo to permit their use as carriers of oxidizing equivalents.