Distribution and properties of receptors for gonadotropin-releasing hormone (GnRH) were analyzed in the brain of adult male rats. Binding of the iodinated GnRH agonist Des-Gly10-(D-Ala6)-GnRH ethylamide was studied in hippocampus and anterior pituitary using three convergent approaches: quantitative autoradiography on frozen tissue, binding to fresh slices, and binding to crude membrane preparations. In all cases, binding was specific, saturable, and time, pH, and temperature dependent. Quantitative autoradiography revealed that the density of binding sites was high in the stratum oriens and stratum radiatum of the CA1-CA4 regions of Ammon’s horn. The pyramidal cell layer was faintly labelled. Binding was almost undetectable in the dentate gyrus. The highest density of sites (Bmax = 11.6 + 1.0 fmol/mg protein) was observed in the stratum radiatum of the CA3 region. Under the same conditions the value obtained for pituitary tissues was 20.7 + 2.8 fmol/mg protein. Analysis of saturation curves indicated only one class of high-affinity sites for the hippocampus (CA3; Kd = 0.28 + 0.03 nM) and for the pituitary (Kd = 0.29 ± 0.08 nM). Both native GnRH and GnRH antagonist were potent competitors of binding. Fresh slices and membrane preparations from whole hippocampus confirmed these autoradiographic data and yielded affinity constants of 0.28 + 0.01 and 0.52 + 0.08 nM, respectively. In addition, a very high binding density was present in the amygdaloid complex, while binding was barely detectable in the hypothalamus. These results demonstrate that high densities of specific GnRH receptors are present in areas concerned with the regulation of behavioral functions. GnRH may thus be involved as a neurotransmitter in the integration of endocrine and behavioral reproductive process