Tanaka, T., Y. Ohira, M. Danda, H. Hatta, and I. Nishi. Improved fatigue resistance not associated with maximum oxygen consumption in creatine-depleted rats. J. Appl. Physiol. 82 (6): 1911-1917, 1997.Effects of feeding of either creatine or its analog -guanidinopropionic acid (-GPA) on endurance work capacity and oxygen consumption were studied in rats. Resting high-energy phosphate contents in hindlimb muscles were lower in the -GPA group and higher in the creatine group than in controls. The glycogen contents in resting hindlimb muscles of rats fed -GPA were significantly higher than those in controls. The endurance run and swimming times to exhaustion were significantly greater (32-70%) in the -GPA group than in the control and creatine groups. However, there were no beneficial effects on the maximum oxygen consumption (O_{2 max}) and oxygen transport capacity of blood by the feeding of -GPA. None of these parameters were significantly influenced by creatine supply. Both maximum exercise time and O_{2 max} in the -GPA group were not changed by normalization of glycogen levels. The activities of mitochondrial enzymes in skeletal muscles were higher in the -GPA group than in the controls. Thus endurance capacity is improved if the respiratory capacity of muscles is increased, even when the contents of high-energy phosphates in muscles are lower. Increased endurance capacity was not directly associated with the elevated levels of muscle glycogen, oxygen transport capacity of blood, or O_{2 max}.