Vasoactive intestinal polypeptide and acetylcholine are responsible for neurovasodilation in the head of the cat.

There is reason to believe that the dilator innervation to blood vessels of many of the tissues of the head may be part of a common outflow from the central nervous system. Part of the effect of its activation is due to the release of acetylcholine and evidence is presented that, in addition, vasoactive intestinal polypeptide (VIP) release also plays a role. The non-cholinergic part of the dilator response to nerve activity can in a series of blood vessels be correlated with the level of VIP in their wall and is selectively reduced by VIP antiserum. In addition, VIP has suitable characteristics as a vasodilator. Whereas substance P which is present in the walls of many arteries of the head, does not qualify as a putative dilator transmitter. Despite the fact that the physiological role of dilator innervation to the circulation as a whole is not understood, the powerful consequences of the activation of the system in particular blood vessels suggests that this innervation should be seriously considered when studying the regulation of peripheral resistance.