 1. To characterize endothelial function in vivo the effect of vasodilatating substances is measured in the forearm circuit. The substances is administered via a thin catheter placed in the Brachial artery near the elbow in local anesthesia. Once the vessels dilate, the blood flow in the forearm will increase and the flow change is a measure of the vasodilatory capacity of that substance. The change is measured with classical venous occlusion plethysmography. The method described is established at Aarhus University Hospital and has been used for several decades to illustrate the effect of vasoactive drugs on the human circulation, meaning it is a proven and well documented method. 