The present study is a phase I, single-centre, double-blind, randomized, cross-over (3 treatments, 3 treatment periods and 6 sequences), stratified (background medication: metformin vs. diet-only), placebo-controlled study, comparing periods lasting 6-9 days on treatment with repeated doses of vildagliptin, sitagliptin, or placebo, with wash-out periods between treatment periods lasting 21 days minimum.