Five interrelated experiments were conducted to study aspects of the mechanisms underlying the genetic variation of mating behavior in bidirectionally selected lines of chickens. Birds utilized included the Athens-Canadian random bred population and lines that had undergone 13 and 14 generations of selection for high and low cumulative number of completed matings.

The enzymatic reduction of tetrazolium by Δ5,3β hydroxysteroid dehydrogenase was employed to identify the sites of steroid biosynthesis in developing embryos of all three lines. The histochemical procedure proved insufficiently precise for determining the onset of steroidontogenesis or for quantifying steroid biosynthetic rates.

Mating behavior was studied in capons, in males receiving testicular autotransplants, and in intact males. Androgen therapy was utilized and consisted of intramuscular injections of 12.4, 44.8, 89.6, or 179.2 mg. testosterone cypionate (TC).

Mating behavior of intact males, as measured by the cumulative number of courts, mounts, treads, and completed matings, was greater in the high mating than in the low mating line with unselected control population being intermediate and significantly different from each of the selected lines.

TC injections into capons increased the frequencies of mating behavior traits over the levels attained by birds receiving carrier oil only; 89.6 mg. TC failed to increase the mating behavior frequencies over those attained by capons receiving the 44.8 mg. TC level. Caponization decreased mating activity while testicular autotransplantation increased mating activity in the high mating line.

In caponized chickens TC injections elicited courting behavior in all lines and sexual behavior in the high mating and control lines. In the low mating capons TC dosages (12.4 - 179.2 mg.) did not significantly increase the frequency of sexual behaviors. No apparent line differences were found in the time period required after TC therapy for attaining maximal behavioral responses. The duration of mating activity was affected by TC dose levels and line differences in the duration of behavioral responses were indicated.

Based on the results of this series of interrelated experiments and because of the insensitivity to exogenous androgens demonstrated by the mating lines, it was suggested that selection in the high and low mating lines has been directed primarily towards an inhibitory mating center and secondarily towards a stimulatory mating center. The inhibitory mating center was presumed to operate independent of the steroid hormones, explaining the lack of response to exogenous androgens.