Racemization of aspartyl (Asp) residues in peptides and proteins has been considered to be a nonenzymatic chemical reaction which occurs via succinimide formation. However, it has not been known yet what conditions in living body accelerate the inversion of the L- to the D-form. Here, we examined the effect of ultraviolet (UV) exposure or oxidative stress on the formation of D-Asp residues in the elastin mimic peptides with or without heat treatment. As a result, UV exposure did not affect the D-Asp formation in peptides. On the other hand, the amount of D-Asp in heat-treated peptide solution at the same time as addition of HO(.) radical, H(2)O(2), and lipid peroxide was significantly increased. These results indicate that the inversion rate to D-form of Asp residues in skin elastin is accelerated by generation of reactive oxygen species (ROS), and that oxidative stress might be closely related to D-Asp formation in aging proteins.