As the glycation/glycoxidation hypothesis for the genesis of diabetic complications is achieving widespread acceptance, much attention is being paid to the role of low molecular weight advanced glycation (AGE) adducts, as second generation glycating agents. We set out a study with the objective of attesting the presence of increased amounts of AGE-peptides in the circulation of streptozotocin-induced diabetic rats and to determine the nature of the plasma proteins which are main targets for advanced glycation. AGE (Ex 370/Em 440 nm) and pentosidine fluorescence (Ex 335/Em 385 nm) were significantly higher in plasma from diabetic rats after only one month of hyperglycemia as compared to controls (35 +/- 7 vs 25 +/- 2 AU, p< 0.05 and 54 +/- 14 vs 27 +/- 3 AU, p< 0.01 respectively). AGE-peptides (<10 kDa) were more than two-fold higher in diabetic animals. Immunoblots after SDS-PAGE of plasma proteins showed that AGE-IgG displayed a selective predominant increment in the same animals. When native rat IgG was incubated in the presence of AGE-peptides isolated from diabetic animals, AGE modification was already apparent after only 24 h of incubation, and was particularly important for light chains. AGE-immunoreactive light chains displayed an apparent increase in molecular weight. Aminoguanidine prevented, while copper enhanced AGE binding to IgG light chains. Our data validate the streptozotocin-induced diabetic rat as a model reproducing the presence of circulating AGE-peptides, give evidence that IgG are preferential targets for advanced glycation in plasma and suggest that this modification, mediated by AGE-peptides, can be prevented by aminoguanidine.