The levator ani of the female adult rat is greatly atrophied in comparison to the same muscle in males. In the present study, the female levator ani was, nevertheless, found to contain type IIb myosin isoforms similar to those contained in the male muscle. These adult type isoforms were, however, synthesized later in the female than in the male levator ani: the half-transition times of the myosin transition curve were 20 days postnatal in the male and 35 days postnatal in the female. The transition curves for castrated and uncastrated male rats were the same. Thus, the presence of male gonadal hormones apparently did not affect the myosin transition.