Dipeptidyl peptidase IV is a very specific protease that attracts growing scientific interest during the last few years. The enzyme has been purified to homogeneity from various human tissues. Histochemically, this protease is found at certain border lines of many organ compartments, as in the proximal tubuli of kidney, in the bile canaliculi of liver, in the capillary endothel, or in the myofibroblasts of placenta. In the blood, especially T-helper lymphocytes contain this enzyme. Dipeptidyl peptidase IV seems to be predestinated for regulatory functions, because it is located on the outer membranes of these cells. The peptidase very specifically degrades substance P. Thus, it is discussed whether the system substance P/dipeptidyl peptidase IV is involved in the regulation of blood pressure, especially in the placenta. On the other hand, the specific attack of the peptidase on the alpha-chain of monomeric fibrin considerably reduces the clotting potency of these molecules. Therefore, dipeptidyl peptidase IV may also be involved in the regulation of blood coagulation in intact vessels, especially because the capillary endothel is lined with this enzyme. The plasma zinc concentration seems to influence the peptidase activity. An increase in plasma zinc stimulates various factors that promote blood clotting.