Because of their remarkable longevity, lens crystallins undergo a substantial amount of glycation (non-enzymatic glycosylation) during diabetic hyperglycemia. These post-translational modifications have the potential to disrupt the structural and functional properties of the lens crystallins and contribute to the formation of cataracts. Streptozotocin-induced diabetic rats were used to study the relationship between glycation of lens proteins and the formation of insoluble high-molecular-weight (HMW) aggregates believed to be responsible for cataract formation. After the onset of diabetes, cataracts developed in about 12- to 13 weeks. The animals were followed in this manner until cataracts developed and for an additional 63 days. Five control and five diabetic rats were killed every 3 weeks and lenses removed. Levels of glycated protein and glycated amino acids in lenses from each animal were examined by affinity chromatography. In addition, the changes in crystallin composition and development of HMW aggregates were monitored by molecular-sieve HPLC techniques. As diabetic hyperglycemia continued there was a linear increase in glycated protein in both the soluble and insoluble fractions. This increase was paralleled by an increase in the soluble HMW and insoluble HMW aggregates. Other changes included a decrease in reactive sulfhydryls which indicates an increase in disulfide bond formation. The gamma-crystallin levels also decreased in a linear fashion during the hyperglycemic pre-cataract and cataract stages. It appears that the glycation of lens crystallin, the disappearance of reactive sulfhydryls and the formation of HMW aggregates are interrelated.