Advanced glycation endproducts (AGEs) are believed to play important roles in the molecular basis of long-term diabetes complications. In the present study we have revealed direct reactivity of AGEs towards kidney tissue structures. Bovine serum albumin, modified by advanced glycation and tagged with colloidal gold (AGE-BSA-gold) was applied to renal tissue sections from normal and long-term diabetic rats by a post-embedding procedure at the electron microscopic level. The AGE-BSA probes were characterized by fluorometry, spectrophotometry, free amines analysis, measurement of early glycation products, electrophoresis, and isoelectrofocusing. When applied to renal tissue sections, AGE-BSA-gold exhibited strong labeling on nuclei. At the extracellular level, glomerular basement membranes and mesangial matrix were labeled. Probes having a higher content of AGEs yielded more intense labeling. Control experiments demonstrated the specificity of the labeling obtained. Quantitative evaluation indicated little variation in reactivity among tissues from rats of different ages. However, a significant increase in reactivity was established for tissues from streptozotocin-induced long-term diabetic rats. This study thus provides direct evidence that soluble AGEs bind to cell nuclei and to structures implicated in the development of diabetic glomerular nephropathy, demonstrating higher AGE reactivity in tissues from diabetic animals.