This study investigated the effects of either growth hormone or thyroxine on muscle fiber atrophy caused by hypophysectomy in male Wistar rats. Muscle fiber size is reported as equivalent circle diameter (ECD) in transverse section of fresh-frozen gastrocnemius muscle. Three months post-hypophysectomy type 1 (slow twitch) and type 2 (fast twitch) muscle fibers were smaller (p < 0.01) than those in intact controls as shown previously in this laboratory. The administration of human growth hormone (GH) (50mIU/100g body weight/day) to hypophysectomized (hypox) rats for 42 days, stimulated body growth, restored 42% of the lost gastrocnemius muscle weight (p < 0.05) and 36% of the lost type 1 fiber size, (p < 0.01), but had no effect on type 2 fiber size. Treatment of hypox rats with physiological doses of thyroxine (5 micrograms T4/100g body weight/alternate day) for 42 days did not affect body growth, gastrocnemius muscle weight or type 1 fiber size, but reduced the size of type 2 fibers (p < 0.01). Thyroxine prevented the decline in the percentage of type 2 fibers which occurs after hypophysectomy. This unique observation suggests that thyroid hormone regulates the proportion of different fiber types in the gastrocnemius muscle. Thus, in hypox rats, GH promoted growth of type 1 or slow twitch muscle fibers needed to support the increasing weight of the growing body. Physiological doses of T4 did not stimulate growth, but caused further atrophy of of type 2 fibers possibly while providing fuel for rapid movement.