Glucose has been shown to react nonenzymatically in vitro with DNA, to form products with spectral properties similar to those observed with the nonenzymatic glycosylation of proteins in vivo. The incubation in vitro of glucose or glucose 6-phosphate with f1 phage DNA results in a time- and concentration-dependent loss of transfection efficiency. It has also been shown that incubation in vitro of pBR322 DNA with glucose 6-phosphate prompts a loss in transformation capability as well as gross DNA alterations. In the present communication, we have investigated a model reaction of glucose 6-phosphate with the amino groups of lysine to form reactive intermediates which are capable of forming covalent adducts with DNA. The preincubation of glucose 6-phosphate and [3H]lysine leads to a time- and concentration-dependent formation of reactive intermediates. These intermediates, which accumulate with time, can subsequently react with single- or double-stranded DNA to form acid-stable complexes. Studies done with synthetic polynucleotides suggest low reactivity of the intermediate with thymidine. The formation of the reactive intermediates is saturated by the addition of excess unlabeled lysine. Once formed the intermediates are insensitive to the addition of aminoguanidine and to reduction by sodium borohydride. The chemical reactions between sugars and lysine reported here and the reactivity of that product with DNA provide a model for exploring the classes of DNA damage that may contribute to the loss of DNA function during aging.