Female--castrated and male-sham operated rats were subjected to five hours of swimming. It was found that estradiol administered to female rats promotes a supercompensatory pattern of glycogen recovery in the myocardium at the end of the fifth hour of exercise. The hormone markedly delayed the glycogen mobilization from the liver during the first hour of swimming. Glycogen level in m. biceps femoris was essentially unchanged by estradiol both at rest and during exercise. Estradiol signifcantly increased glycogen level in m. masseter of resting rats. Hypoglycaemia developing at the end of the fifth hour of exercise was less pronounced in the estradiol--treated female rats than in the animals from other groups. It may be concluded that estradiol exerts a "sparing" effect on carbohydrate reserves during exercise in rats.