 Creatine and phosphocreatine are the major intracellular buffers for adenosine triphosphate. They also function as the energy shuttle for high energy phosphate from the mitochondrial site of production to the cytoplasmic site of utilization. In addition, creatine has been shown to affect satellite cell proliferation and differentiation and increase cell mitotic activity during compensatory hypertrophy in rat skeletal muscle. Creatine is synthesized from three amino acids: glycine, arginine and methionine. Although creatine metabolism has been studied extensively in the rat, the data regarding the kinetics of creatine and their regulation in humans are limited. This is in part due to a large and slow turning over pool of creatine and lack of good tracer methods. Using a recently developed stable isotopic tracer method by us, we propose to quantify the rate of synthesis of creatine and its regulation in healthy subjects and in those with large skeletal muscle mass as a result of endurance training. In addition, the response to oral creatine administration on creatine kinetics will be quantified. The site of regulation will be identified by the relative rate of appearance of tracer glycine in guanidinoacetic acid and creatine. It is hypothesized that the fractional rate of synthesis of creatine will be higher in endurance trained subjects, and that creatine will suppress the rate of synthesis of creatine. Since creatine is critical for the maintenance of skeletal muscle mass and satellite cell proliferation, these data will provide the normative physiological data for future studies of subjects with manifest sarcopenia, such as aging, and those with chronic diseases, e.g. cirrhosis of liver. These data will form the basis for the studies of creatine metabolism in neurodegenerative disorders where creatine is being increasingly used as a therapeutic agent. 