It has been deduced (Lancaster, Proc. Natl. Acad. Sci. USA 91 (1994) 8137-8141), from a consideration of Fick's law of diffusion, that the very effective scavenging of nitric oxide (NO) by haemoglobin in red blood cells prevents any NO from endothelial cells migrating outwards into vascular smooth muscle. This conclusion has led some authors to suggest that endothelium-derived relaxing factor (EDRF) is not free NO. We have reconsidered the application of Fick's law to the migration of NO in the vasculature, making allowance for the reaction of NO with guanylate cyclase and for the layer of red blood-free plasma next to the endothelium. The source of NO is taken as an infinite cylinder. Calculations for vessels of various diameters indicate that a substantial amount of NO migrates outwards in spite of very effective scavenging by haemoglobin and that the relative amount of NO migrating outwards depends upon the radius of the vessel. The view that locally produced NO is not responsible for vascular dilation has not been sustained.