Neuropeptide Y (NPY) is a 36 amino acid peptide that was first isolated from porcine brain (Tatemoto et al. 1982). There is considerable evidence that NPY co-exists with noradrenaline in a subset of peripheral sympathetic neurons (Lundberg et al. 1982). It has been shown to contract blood vessels and may mediate sympathetic vasoconstrictor responses that are resistant to a-adrenoceptor blockade (Lundberg & Tatemoto 1982). In addition, NPY has been shown to markedly enhance the vasoconstrictor responses to several agents, including noradrenaline (NA) in peripheral arteries (Edvinsson et al. 1984), and to inhibit the release of NA from the field stimulated vas deferens (Lundberg & Stjarne

In the present series of experiments we have examined the possibility that NPY has an effect that is similar to that of a,-adrenoceptor agonists like clonidine, that is, that it inhibits the formation of cyclic AMP. T o test this hypothesis we have stimulated the adenylate cyclase by the diterpene forskolin (Seamon & Daly 1983). The results indicate that NPY is a very potent inhibitor of forskolin-stimulated adenylate cyclase. It is suggested that this may be the mechanism underlying some of the biological effects of the peptide.