The specific binding of the chloramine-T iodinated neuropeptide Y (125I-NPY) to membranes from rat cerebral cortex was investigated using equilibrium binding and kinetic methods. The equilibrium binding of 125I-NPY at 37 degrees C was characterized by a Kd value of 0.38 nM. The receptor densities in the cerebral cortex, hypothalamus and cerebellum were 0.45 pmol/mg, 0.47 pmol/mg and 0.04 pmol/mg protein respectively. The binding site for 125I-NPY was sensitive to treatment with proteolytic enzymes and thiol reagents. The binding showed a sharp optimum at pH 7-7.7 and was inhibited by increasing concentrations of Mg2+.