Although the rat's 'levator ani' muscle has been widely used for screening and evaluating nonandrogenic anabolic steroids, there has been some confusion about the nature of the muscle itself. To clarify this situation, the anatomical literature was searched and numerous dissections were performed. It was found that the name 'levator ani' is incorrect and misleading. This muscle is not homologous to the one called levator ani in man, cat, and dog, but is actually the dorsal bulbocavernous, a muscle peculiar to rodents. It attaches to the bulb of the urethra; it is absent in the female rat; and, unlike the true levator ani, it is clearly part of the male reproductive system. To determine whether the masculinizing action of anabolic steroids is confined to this particular organ, its response to three such steroids was compared with the responses of several other genital organs. Although these steroids had little effect on the accessory sex glands, their strong action on the dorsal bulbocavernous was accompanied by similarly accelerated growth of all other muscular and erectile parts of the male genital system, including the penis. These results cannot be understood in terms of the traditional dichotomy of androgens and nonandrogens. A third category of quasi-androgens is needed, to accommodate synthetic steroids with some, but not all, of the masculinizing actions of testosterone.