We injected adult female rats with 0.25 mg/100 g body weight of testosterone to test the sensitivity to androgens of the levator ani muscle (LAM). Testosterone induced marked hypertrophy characterized by fiber size enlargement, but did not increase the number of fibers. Morphological observations and quantitative data indicated that hypertrophy was accompanied by satellite cell proliferation between Days 1 and 3 after testosterone treatment, and by an increase in the number of myonuclei, which started between Days 2 and 3 after treatment. On Day 30, this increase reached 80% of the initial number of myonuclei. The new myonuclei seemed to result from satellite cell proliferation and from the fusion of some of them with preexistent muscle fibers. These results strongly suggest that testosterone-induced cell proliferation might have a role in developing the sexual dimorphism of the LAM. They also indicate the need to reconsider the currently accepted notion that sexual dimorphism is owing to female LAM involution.