Structural changes in various penile tissue from rats with differing circulating androgen levels were analyzed with light and scanning electron microscopy. Adult males were castrated and daily injected for 6 weeks with one of four physiologic amounts of testosterone propionate (TP). Results indicated that epithelium, subepithelial connective tissue, and the papillae protruding from the surface of the glans penis responded, more or less, in a dose-dependent fashion to the steroid. The least androgen-sensitive penile tissue was the epidermal stratum corneum, more sensitive was the maligiphian layer, and more sensitive still was the subepithelium. The penile papillae and their follicles were the most androgen-sensitive tissue. Specifically, the high testosterone, 400 μg and 800 μg TP, males experienced papillae that were thicker, taller, more densely arranged, and more highly keratinized than the lower testosterone, 100 μg and 200 μg TP, males. The function of the papillae may be to provide the female with the mechanical stimulation necessary to ensure a proper hormonal milieu for successful impregnation.