In endothelial cells cultured under high glucose conditions, methylglyoxal is the major intracellular precursor in the formation of advanced glycation endproducts. We found that endothelial cells incubated with 30 mM d-glucose produced approximately 2-fold higher levels of methylglyoxal but not 3-deoxyglucosone and glyoxal, as compared to 5 mM d-glucose. Under hyperglycaemic conditions, the methylglyoxal-arginine adduct argpyrimidine as detected with a specific antibody, but not N(e)-(carboxymethyl)lysine and N(e)-(carboxyethyl)lysine, was significantly elevated. The glyoxylase I inhibitor HCCG and the PPARgamma ligand troglitazone also increased argpyrimidine levels. Increased levels of argpyrimidine by glucose, HCCG and troglitazone are accompanied by a decrease in proliferation of endothelial cells. A 27 kDa protein was detected as a major argpyrimidine-modified protein. With in-gel digestion and mass spectrometric analysis, we identified this major protein as heat-shock protein 27 (Hsp27). This argpyrimidine modification of Hsp27 may contribute to changes in endothelial cell function associated to diabetes.