Accumulation of advanced glycation end products (AGEs) on tissue proteins increases with pathogenesis of diabetic complications and atherosclerosis. Here we examined the effect of peroxynitrite (ONOO(-)) on the formation of N( epsilon )-(carboxymethyl)lysine (CML), a major AGE-structure. When glycated human serum albumin (HSA; Amadori-modified protein) was incubated with ONOO(-), CML formation was detected by both enzyme-linked immunosorbent assay and high-performance liquid chromatography (HPLC) and increased with increasing ONOO(-) concentrations. CML was also formed when glucose, preincubated with ONOO(-), was incubated with HSA but was completely inhibited by aminoguanidine, a trapping reagent for alpha-oxoaldehydes. For identifying the aldehydes that contributed to ONOO(-)-induced CML formation, glucose was incubated with ONOO(-) in the presence of 2,3-diaminonaphthalene. This experiment led to identification of glucosone and glyoxal by HPLC. Our results provide the first evidence that ONOO(-) can induce protein modification by oxidative cleavage of the Amadori product and also by generation of reactive alpha-oxoaldehydes from glucose.