Endothelium dependent relaxation of isometrically mounted rabbit aortic strip preparations was rapidly inhibited by human plasma at dilutions down to 1:1000. Gel filtration and ion exchange chromatography were used to demonstrate that this inhibitory activity was present in fractions containing haptoglobin. Purified haptoglobin itself possessed no inhibitory action against endothelium dependent relaxation, but the haptoglobin-haemoglobin complex did, consistent with the documented ability of haemoglobin to inhibit this phenomenon. The concentration of haemoglobin normally bound to haptoglobin is sufficient to account for the inhibitory properties of human plasma. This suggests that endothelium derived relaxing factor exerts no downstream intravascular effect in vivo and thus that its physiological dilator role is that of a local autocoid acting on subjacent smooth muscle.