A novel peptide, calcitonin gene-related peptide (CGRP), has been predicted to result from alternative processing of the primary RNA transcript of the calcitonin gene in the rat^1,2. Several lines of evidence suggest that CGRP is a transmitter in the central and peripheral nervous system^2–4. Human CGRP has been isolated and characterized⁵, and shown to have potent effects on the heart⁶. The observations presented here indicate that human and rat CGRP also have potent effects on blood vessels. Intradermal injection of CGRP in femtomole doses induces microvascular dilatation resulting in increased blood flow, which we have detected in the rabbit by using a ¹³³Xe clearance technique^7,8. In human skin, CGRP induces persistent local reddening. Microscopic observation of the hamster cheek pouch⁹in vivo revealed that topical application of CGRP induces dilatation of arterioles. Furthermore, CGRP relaxes strips of rat aorta in vitro by an endothelial cell-dependent mechanism. Therefore, we suggest that local extravascular release of CGRP may be involved in the physiological control of blood flow and that circulating CGRP may contribute to hyperaemia in certain pathological conditions.