Chronic denervation of m. extensor digitorum longus in rats significantly increase the heat resistance of muscle fibers. This effect does not result from muscle disuse, since tenotomy does not affect the level of heat resistance of muscle fibers. After bilateral adrenalectomy, the thermostability of muscles remains unaffected. The data obtained are discussed in relation to the problem of coupled changes in thermostability and electrogenesis of muscles fibers in animal evolution.