The aim of this study was to investigate the effects of aging on glucose metabolism after oral glucose challenge in aged dipeptidyl peptidase IV (DPP-IV) positive (+) Fischer 344 (F344), DPP-IV deficient (-) F344 and DPP-IV(+) Wistar rats and to determine the effect of a DPP-IV inhibitor NVP-DPP728 (1-[2-[(5-cyanopyridin-2-yl)amino]ethylamino]acetyl-2-cyano-(S)-pyrrolidine monohydrochloride salt) on glucose tolerance in aged rats. Aging caused a decrease in early insulin response after an oral glucose challenge in aged Wistar or DPP-IV(+) F344 rats, but not in aged DPP-IV(-) F344 rats, compared with young control groups. Glucose tolerance after an oral glucose challenge in aged DPP-IV(-) F344 rats was better than in aged DPP-IV(+) F344 and Wistar rats associated with the preservation of the early insulin response. NVP-DPP728 improved the glucose tolerance after an oral glucose challenge by potentiating the early insulin response throughout the inhibition of plasma DPP-IV activity in aged DPP-IV(+) Wistar and F344 rats. In contrast, NVP-DPP728 did not affect the glucose tolerance after an oral glucose challenge in aged DPP-IV(-) F344 rats. These results indicate that treatment with NVP-DPP728 ameliorated glucose tolerance in aged rats by the direct inhibition of plasma DPP-IV activity and presumably the subsequent increase in endogenous incretin action.