The development of bombesin/gastrin-releasing peptide (BN/GRP) binding sites was determined in the rat brain. Using rat brain homogenate, the density of radio-labeled peptide binding sites increased dramatically around birth, remained constant during the first post-natal week, and then increased again between Postnatal (P) Days 7 and 10; the density of binding sites was similar in P10 and adult animals. Rat brain homogenates derived from P1, P7, or P10 animals bound (125I-Tyr4)BN with high affinity (Kd = 2-4 nM) to a single class of sites (Bmax = 11, 17, and 51 pmol/mg protein, respectively). BN, GRP, and GRP18-27 inhibited specific radiolabeled peptide binding to rat brain homogenate with high affinity using P1, P3, P7, or P14 animals, whereas GRP1-16 was inactive. These data indicate that the C-terminal of BN or GRP is essential for high-affinity binding activity in the rat brain during development. In vitro autoradiography revealed high (125I-Tyr4)BN grain densities in the anterior olfactory nucleus, olfactory tubercle, nucleus accumbens, central medial thalamic nucleus, and hippocampus. Moderate grain densities were observed in the parietal cortex, periventricular hypothalamic nucleus, central gray, inferior colliculus, pontine reticular, and gigantocellular reticular nuclei in P1, P3, and P7 rat brains. In P14 and adult animals, relative to P1, P3, or P7 rat brains, the density of binding sites increased significantly in the parietal cortex. The (125I-Tyr4)BN grain density was not significantly changed in the anterior olfactory nucleus, periventricular nucleus of the hypothalamus, olfactory tubercle, hippocampus, dentate gyrus, central medial thalamic nucleus, and nucleus accumbens but decreased significantly in the central grey, inferior colliculus, and pontine reticular nucleus. These data indicate that there is a redistribution of BN/GRP binding sites in the rat brain during development.