Purpose: OP-lysine, a glycation product of lysine residues of proteins, has been reported to be formed with glyceraldehyde and glycolaldehyde as precursors in the lens, and has been suggested to play a role in senile cataracts. However, there has been no reliable information regarding the content of glyceraldehyde in tissues. This study determined the glyceraldehyde levels in the lenses of normal and diabetic rats. Methods: Glyceraldehyde was derivatized to a fluorescent compound, and the compound was then quantified by high-performance liquid chromatography. Results: The lens glyceraldehyde levels in normal and diabetic rats were 0.75 ± 0.06 and 1.26 ± 0.21 nmol/g wet weight (means ± standard deviations of 6 animals, p < 0.01), respectively. Isolated rat lenses accumulated a higher level of glyceraldehyde when cultured for 6 days in 25.5 mM glucose than when cultured in 5.5 mM glucose. Conclusions: Glyceraldehyde was found to be present in the lens and was increased in diabetes mellitus. OP-lysine is thus likely to be a potential risk factor for senile and diabetic cataracts.