Dimethyl fumarate (DMF) is used successfully to treat psoriasis. In spite of its proven clinical efficacy, the mode of metabolism and the pharmacodynamics of DMF are still not completely understood. Some previous studies have indicated that orally applied DMF for a considerable part is quickly hydrolysed to methylhydrogen fumarate (MHF) at basic pH conditions as present in the upper intestine, especially in the presence of biological fluids containing esterases. On the other hand it was shown that DMF due to its high lipophilicity rapidly penetrates into cells and may thus at least in part be absorbed after po application without being hydrolysed. On the other hand, no detectable amounts of DMF were hitherto found in plasma samples after po administration. In order to shed light on possible further routes of presystemic metabolism of DMF, studies on the reactivity towards glutathione (GSH) were carried out. GSH is present in millimolar concentrations in almost all cells. DMF due to its nature as an alpha,beta-unsaturated carboxylic acid ester can react spontaneously with thiols via a Michael-type addition. It could be shown that DMF reacts at high rates under near-physiological conditions. Studies on the reaction kinetics at pH 7.4 show that GSH addition proceeds rapidly to yield a 1:1 mixture of both diastereomeric 2-(S-glutathionyl)-succinic acid dimethyl esters. MHF under identical conditions was shown to react with GSH as well leading to a mixture of four products (2 diastereomeric pairs). However, MHF reacted at a much lower rate. The structures of all thiol conjugates were confirmed unambiguously by extensive NMR measurements. GSH conjugates and the corresponding mercapturic acids on grounds of the high spontaneous reactivity observed may be expected to be major metabolites of unhydrolysed DMF which makes its way into enterocytes. On the other hand, MHF, due to its slow reaction with GSH, may have higher chances than DMF to react with more essential thiol groups in macromolecules.