Four experiments were conducted in an effort to explain further the genetics and physiology of sexual behavior in chickens and Japanese quail. The populations used in this research included lines that had undergone over 20 generations of divergent selection for high and low mating frequency and the random bred control population from which the selected lines originated. In the first experiment, fertility comparisons under natural and artificial mating situations were made over time among male chickens from the selected and control lines. Although there were highly significant differences among the lines for mating frequency, total fertility was similar among lines when mating was by either natural or artificial means. Significantly more days were needed to reach peak fertility when mating was by natural rather than artificial means. This difference may be attributed to male-female interactions whereby all females in a flock would not be fertilized on the same day in natural mating situations. This same reasoning could explain the significant differences noted between mating situations for duration of fertility. The second experiment examined the mode of inheritance of mating behavior and testosterone levels in chickens using the selected lines plus reciprocal crosses among these lines. No differences among mating combinations were found for circulating testosterone levels. In all cases, androgen titers appeared to be of a sufficient magnitude to influence mating behavior. Heterotic effects were found for mating behaviors in cross-pureline comparisons suggesting that nonadditive genetic variation influences the thresholds for mating in the fowl. Electroencephalographic effects of mating behavior of the selected lines were studied in the third experiment. There was an inverse relationship between number of peaks and voltages per peak, with differences in the number of peaks being significant among lines, implant locations and behavioral situations. Highly significant differences were found among lines for all voltage measurements with the control line having the highest voltage and the low mating line having the lowest voltage. These observations were discussed in the context of their effects on inhibitory and stimulatory mating centers. The genetics of mating frequency in male Japanese quail was studied in the fourth experiment utilizing replicated lines selected for high or low mating frequency and the randombred control line that served as the base population for the selected lines. Comparisons involved the purelines, F₁, F₂ and backcross generation progeny. The results indicate that the primary heritable variation for mating frequency in this species is primarily additive. Correlated responses of cloacal gland size and relative aggressiveness to selection for mating frequency are discussed in the context of alterations in physiological and behavioral responses. / Ph. D.