Steroid autoradiography was undertaken to determine the neuroanatomical loci which might be involved in the activation of steroid-sensitive behaviors in the Japanese quail (Coturnix japonica). Male and female quail were either surgically gonadectomized or photically regressed and implanted with androgen or estrogen to restore normal sexual and courtship behavior. After gonadectomy or implant removal, each quail was injected with 250 µCi of [3H]-testosterone (3H-T), [3H]-estradiol (3H-E2), or [3H]-dihydrotestosterone (3H-DHT), sacrificed, processed for autoradiography, and the telencephalon, diencephalon, mesencephalon, and rhombencephalon were examined for labelled cells. Following 3H-T or 3H-E2 injection and autoradiography, labelled cells were found in nucleus septalis lateralis (SL), nucleus preopticus medialis (POM), nucleus paraventricularis (PVN), regio lateralis hypothalami (LHy), nucleus inferior hypothalami (IH), nucleus infundibuli (IN), nucleus intercollicularis (ICo), substantia grisea centralis (GCt), nucleus taeniae (Tn), and in the reticular formation near nucleus motorius nervi trigemini (MV). In addition, following 3H-E2 autoradiography, labelled cells were found around nucleus accumbens (Ac). Following 3H-DHT autoradiography, labelled cells were found only in SL, PVN, Tn, LHy, ICo, and CGt. No labelled cells were found in Ac, POM, IH, IN, or MV even after long exposure times. These results suggest that the nuclei labelled following 3Η-E2 but not 3H-DHT administration bind exclusively the aromatized metabolites of T. Since quail show a sex difference in male-typical copulatory behavior in response to E2, labelled cells were counted in POM, LHy, IH, and Tn of male and female quail following 3Η-E2 injection and autoradiography. No sex differences in the number of labelled cells were found in POM, LHy, or IH. Males were found to have more labelled cells than females in Tn. These results show that sex differences in male-typical copulatory behavior are not due to sex differences in the number of cells binding estrogens in POM. The results reported here constitute the most neuroanatomically extensive report of steroid binding cells to date for a galliform brain, the first comparison in a galliform bird of the distributions of cells labelled following injection of 3H-T, 3H-E2, and 3H-DHT and the first analysis of sex differences in numbers of estrogen-binding cells in four nuclei in the avian bra