Background/Aims: Inhibition of dipeptidyl peptidase 4 by vildagliptin enhances the concentrations of the active form of the incretin hormones GLP-1 and GIP. The present study asked whether vildagliptin accentuates glibenclamide induced hypoglycemia or affects endogenous secretion of GLP-1 and GIP after an OGTT. Methods: 16 healthy male subjects were studied on four occasions after an overnight fast in a double-blind, 4-way crossover study. In random order, vildagliptin [100 mg od]) or placebo (P), with and without glibenclamide (5 mg od), was administered 30 min prior to 75 g oral glucose. Blood was sampled to measure glucose and total (sum of active and inactive) GLP-1 and GIP. Statistical evaluation was done using repeated measures-ANOVA. Results: Glibenclamide provoked hypoglycemia (1.9 mM), but this was not accentuated by the simultaneous administration of vildagliptin (p = 0.25). The integrated incremental responses of total GLP-1 were reduced by vildagliptin by 72% (with glibenclamide) and by 48% (without glibenclamide) (effect of vildagliptin: p<0.0001; glibenclamide: p = 0.31; interaction: p=0.26). Similarly, integrated incremental responses of total GIP were reduced by vildagliptin by 26% and by 21%, with and without glibenclamide respectively (vildagliptin: p = 0.017; glibenclamide: p = 0.44; interaction: p = 0.69). Conclusions: Sulfonylurea-induced hypoglycemia after the oral administration of glibenclamide is not accentuated by the coadministration of vildagliptin. This may be explained by a negative feedback regulation of GLP-1 and GIP secretion that limits the degree to which the active incretin levels are enhanced.