Neuropeptide Y (NPY) is a cotransmitter with noradrenaline in perivascular sympathetic nerves. It constricts some vessels and in others potentiates other vasoconstrictors. NPY produces vasoconstriction and natriuresis in the isolated perfused rat kidney, but induces renal vasoconstriction without natriuresis in the intact rabbit. We have performed radioligand binding studies in rat, rabbit, and human kidney, using 125I-NPY. In rabbit cortex there is a high density of high-affinity binding (Bmax = 560 fmol/mg, Kd = 83 pM). Receptor density in rabbit medulla is approximately 200 fmol/mg, with much higher nonspecific binding. Binding was displaced with high potency by human NPY and peptide YY, weakly by pancreatic polypeptide, and not at all by structurally unrelated peptides. There was no detectable specific binding in rat kidney, and only a very small number of binding sites in human cortex (10 fmol/mg). No specific binding was obtained in human medulla. These results have been confirmed by light microscope autoradiography, which suggests that NPY binding sites in rabbit cortex are localised to the proximal convoluted tubules and vascular smooth muscle. The functional implications of the above species differences remain to be determined.