The effects of sex hormones on rat soleus and caudofemoralis muscles were examined histochemically. The fiber diameter of three types of fibers, Types 1, 2A, and 2B, was larger in the adult male rat than in the female rat. The size of all three types of fibers was increased by ovariectomy, but the effect was most prominent on Type 2A. Orchiectomy resulted in a decrease in the size of Type 2B, but caused no change in Types 1 and 2A. Estrogen administration resulted in a decrease in the size of all types of fibers in both male and female gonadectomized rats. The total number and composition of each fiber type were not affected by gonadectomy or estrogen administration. Thus, the sex hormones exerted their effects on muscle fiber diameter, with female sex hormones having an inhibitory effect and testosterone having a promotive effect, but the fiber type profiles and the total number of fibers were not affected.