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The influence of thyroid hormone depletion and experimental hyperthyroidism on red and white skeletal muscle of the rat during periods of 2, 4 and 8 weeks were studied. Body weight, muscle weight, mitochondrial protein content, and specific activities of the mitochondrial enzymes \g=a\-glycerophosphatedehydrogenase (EC 1.1.99.5) (\g=a\-GPD) and succinate dehydrogenase (EC 1.3.99.1) (SDH) were used as parameters. The largest differences in body weight gain and muscle weight gain (both red and white muscle) in the hypothyroid rats were seen after 8 weeks of T4 treatment. In the hyperthyroid rats the weight of the red muscle and the ratio of the red muscle weight to the body weight increased, whereas the white muscle weight and the ratio of the white muscle weight to the body weight decreased relative to the control animals. In hypothyroid rats the mitochondrial protein content was lowered in both red and white muscle, the specific \g=a\-GPD activity only in the latter. No changes in specific SDH activity were observed in either type of muscle. The hyperthyroid rats showed an increase in the mitochondrial protein content and the specific \g=a\-GPDand SDH activity in the red muscle, whereas no significant changes were observed in the white muscle. The changes in the parameters under study show that the effect of the thyroid state differs in red and white muscle. An explanation for a possibly greater sensitivity of red than of white muscle to thyroid hormones is discussed.The object of this study was to investigate a possible difference between the susceptibility of red and white skeletal muscle of the rat to thyroid hormones. Evidence that thyroid hormones have a different action on red and white 768

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