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CROSS-INNERVATION experiments have convincingly demonstrated the importance of the nerve in determining the differential properties of skeletal muscle myosin. The role of hormones in deciding the phenotypic character of muscle myosin has received considerably less attention than that of the neural factors. Recent studies have shown that short-term alterations in thyroid status results in significant changes in cardiac muscle myosin^1–4. In contrast, some of those investigations also used skeletal muscle, but no apparent effects on skeletal muscle myosin were detected^2,4. But the greater half-life of skeletal muscle myosin would require a longer time than cardiac myosin to manifest these changes⁵. We have studied the effects of longer term hypothyroidism and hyperthyroidism on skeletal muscle myosin. We report here that alterations in thyroid status influence the quality of skeletal muscle myosin. Our …