Studies are in progress to obtain information regarding the etiology of muscle fatigue. In these studies, both the biochemical and physiological changes that occur during the development of fatigue are being investigated. Other studies in progress are designed to elucidate the control mechanisms by which fat and carbohydrate metabolism are regulated during exercise in exercise-trained and untrained animals. In these studies we are obtaining information regarding the mechanisms responsible for the slower rate of carbohydrate utilization in trained, than in untrained, animals performing the same work. The time course of the various adaptions to exercise-training, and the time course of their reversal after cessation of exercise-training, are also under investigation. BIBLIOGRAPHIC REFERENCES: Holloszy, J.O., and Booth, F.W. (1976) Biochemical adaptations to endurance exercise in muscle. Ann. Rev. Physiol. Rennie, M.J., Winder, W.W., and Holloszy, J.O. (1976) A sparing effect of elevated plasma fatty acids on muscle and liver glycogen in the exercising rat. Biochem. J.