Quantitative receptor autoradiography was used to study the neuroanatomical distribution and effects of gonadal hormones on [3H] flunitrazepam binding in the male Japanese quail brain. In gonadally intact quail brains, [3H] flunitrazepam displayed a heterogeneous distribution, with elevated levels in the posterior brain regions such as the stratum griseum et fibrosum superficiale and stratum griseum centrale of the optic tectum. Lower values were observed in the anteriorly located brain sites such as the nucleus septalis (lateralis et medialis), the cortex dorsolateralis and the nucleus lateralis hypothalami. Castration affected [3H] flunitrazepam binding levels in brain areas known to contain gonadal steroid receptors as well as in some areas which were devoid of gonadal steroid receptors. Castration in fact, elevated [3H] flunitrazepam binding in the nucleus preopticus anterior and reduced binding levels in archistriatum dorsalis et ventralis and in the nucleus intercollicularis; all of these areas are known to have gonadal steroid receptors. In two regions which do not contain such receptors, namely the hyperstriatum ventrale and the cerebellum pars granularis, castration increased [3H] flunitrazepam binding. In order to determine whether the gonadal steroid effect is due to changes either in the number of binding sites (Bmax) and or affinity binding state (KD), saturation binding studies were carried out in some of the areas described above in brains of quail which were castrated or castrated and given replacement therapy with testosterone or estradiol for 2 weeks. In keeping with the suppressive effect of the gonads inferred from the castration results described above, testosterone and estradiol decreased the Bmax of [3H] flunitrazepam binding in the hypothalamus preoptic area, while in hyperstriatum ventrale only estradiol produced a significant decrease of the Bmax. We then tested for the effects of GAB A on [3H] flunitrazepam binding to the receptors and found that GABA intensified the depressive activity of gonadal steroid replacement therapy in the hyperstriatum ventrale and in the hypothalamus-preoptic area as shown respectively by the approximate 45 and 40% greater decrease of Bmax. However when the GABA effect on [3H] flunitrazepam binding in the presence of GABA was expressed as ratio, only the hyperstriatum ventrale revealed a significantly enhanced potentiation effect following castration, while a significant suppression of this potentiation effect was obtained by both T and E replacement therapy. These results suggest that a GABA-benzodiazepine receptor interaction might be involved in the regulation of some hormone-mediated cerebral functions in the male quail such as neuroendocrine and sociosexual behavior.