The synthetic hexapeptide, His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (GHRP, Growth Hormone-Releasing Peptide), has no structural similarities with any of the GH-releasing peptides known and its action in releasing GH is by a complementary but yet not clearly defined action on the pituitary as well as hypothalamus. Therefore, in vitro studies have been performed to demonstrate and characterize GHRP binding sites on peripheral membranes of both porcine pituitary and hypothalamus. The membrane binding sites were specific, reversible, saturable and time, temperature, pH and concentration dependent under optimum binding assay conditions. The maximum specific binding was observed between pH 5.0 and 6.0. In the presence of Ca2+ and Mg2+ ions, with or without chelating agents there was a significant reduction in the specific binding. Scatchard analysis of these binding sites using increasing doses of unlabeled GHRP revealed a single low affinity site with a 2.1 x 10(-5) M and 1.7 x 10(-5) M and a maximum number of sites of 10 nmol/mg protein and 5 nmol/mg protein for pituitary and hypothalamus, respectively. It is also observed that (D-Lys3)-GHRP, substance P antagonists and growth hormone-releasing factor analog were potent inhibitors of GHRP binding in both tissues.