Dipeptidyl peptidase-4 (DPP-4) inhibitors prevent degradation of incretin hormones (glucagon-like peptide 1 [GLP-1] and glucose-dependent insulinotropic polypeptide [GIP]), whereas metformin may increase GLP-1 levels. We examined, in a four-period crossover trial, the influence of metformin (2,000 mg/day), sitagliptin (100 mg/day), or their combination, on GLP-1 responses and on the incretin effect in 20 patients with type 2 diabetes, comparing an oral glucose challenge (75 g, day 5) and an “isoglycemic” intravenous glucose infusion (day 6). Fasting total GLP-1 was significantly increased by metformin and not changed by sitagliptin. After oral glucose, metformin increased and sitagliptin significantly decreased (by 53%) total GLP-1. Fasting and postload intact GLP-1 increased with sitagliptin but not with metformin. After oral glucose, only sitagliptin, but not metformin, significantly augmented insulin secretion, in monotherapy and as an add-on to metformin. The incretin effect was not changed numerically with any of the treatments. In conclusion, sitagliptin increased intact GLP-1 and GIP through DPP-4 inhibition but reduced total GLP-1 and GIP (feedback inhibition) without affecting the numerical contribution of the incretin effect. Insulin secretion with sitagliptin treatment was similarly stimulated with oral and “isoglycemic” intravenous glucose. This points to an important contribution of small changes in incretin concentrations within the basal range or to additional insulinotropic agents besides GLP mediating the antidiabetic effects of DPP-4 inhibition.