The neuroendocrinology of masculine sexual behavior was examined in Japanese quail selected bi-directionally for adult mating frequency. Experiments were conducted with brain tissues from functionally castrated, High (HM) and Low Mating (LM) line quail to compare (1) the in vivo aromatization of ³H-testosterone and (2) cytosolic estradiol-binding by in vitro saturation analyses. After in vivo incubation with ³H-testosterone, all radioactivity recovered in brain tissues was in the form of testosterone, dihydrotestosterone, or estradiol. Neither the total ³H nor ³H-testosterone metabolite radioactivity differed upon comparison of the two genetic lines. Of all ³H-testosterone metabolite radioactivity, ³H-estradiol represented 45 ± 6% in the HM line and 46 ± 6% in the LM line, indicating that the line difference in mating frequency was not due to a corresponding difference in aromatase activity. Inasmuch as both the HM and LM line birds actively converted testosterone to estradiol, these results implicated a neural mechanism involving estradiol-receptor interactions. Estradiol-receptor binding parameters were estimated in subsequent experiments by fitting a hyperbolic saturation curve to point measurements of total binding. Using this single-class binding site model, the apparent dissociation constant (K_d) for the estradiol-receptor interaction in the HM line was 0.40 ± .06 nM and the number of specific estradiol binding sites (B_max) was 10.4±.4 fmoles/mg cytosol protein. For the LM line, an apparent Kd of 0.41 ± .20 nM and a B<max> of 9.50 ± 1.0 fmoles/mg cytosol protein were obtained. The similarity between corresponding binding parameters for the divergent mating lines indicated that the number and/or estradiol-binding affinity of a single class of cytosol receptors was also not responsible for the line difference in mating frequency. Though these results confirm that interaction of estradiol with cytosol receptors was not the limiting neuroendocrinological mechanism in the differential expression of masculine sexual behavior in quail, the possible involvement of dynamic interrelationships between cytoplasmic and nuclear estrogen binding was discussed. / Ph. D.