DPP-4 exists in a soluble form in plasma ad as a membrane-bound form in blood vessels and other tissues. The impact of DPP-4 inhibitors on DPP-4 activity has only been evaluated in plasma. We aim to investigate plasma and endothelial DPP-4 activity (i.e. whole-body DPP-4 activity) in patients with type 2 diabetes during different doses of the DPP-4 inhibitor sitagliptin.