In this study the magnitude of non-sympathetic, non-cholinergic neurogenic vasodilation of feline cerebral arteries in vitro was correlated with the extent of innervation by VIP-immunoreactive nerves. Well-innervated arteries underwent nerve-mediated relaxation whereas those that are not supplied with VIP-containing axons did not relax to transmural nerve stimulation. The relaxation of cerebral arteries that are well endowed with VIP-immunoreactive nerves was selectively and reversibly inhibited by VIP-specific antiserum. Substance P-specific antiserum did not affect the dilator responses. We conclude that VIP is a functional neurodilator transmitter in the cerebral circulation.