Phosphocreatine undergoes irreversible cyclization and dehydration to form creatinine at a fractional rate of 0.026 per day, thus forming approximately 2 g creatinine/day in an adult male. This is the amount of creatine that must be provided either from dietary sources or by endogenous synthesis to maintain the body pool of (creatine and) phosphocreatine. Creatine is an amino acid that plays a vital role as phosphocreatine in regenerating adenosine triphosphate in skeletal muscle to energize muscle contraction. Creatine is phosphorylated to phosphocreatine in muscle in a reaction that is catalyzed by the enzyme creatine kinase. This enzyme is in highest concentration in muscle and nerve. Oral administration increases muscle stores. During the past decade, creatine has assumed prominence as an ergogenic (and legal) aid for professional and elite athletes. Most (~ 95%) of the total body creatine-phosphocreatine pool is in muscle (more in skeletal muscle than in smooth muscle) and amounts to 120 g (or 925 mmol) in a 70 kg adult male. Approximately 60-67% of the content in resting muscle is in the phosphorylated form. This generates enough ATP at the myofibrillar apparatus to power about 4 seconds of muscle contraction in exercise. Phosphocreatine reacts with ADP to yield ATP and creatine; the reversible reaction is catalyzed by creatine kinase. phosphocreatine is the chief store of high-energy phosphates in muscle. Thus, this reaction, which permits the rephosphorylation of ADP to ATP, is the immediate source of energy in muscle contraction. During rest, metabolic processes regenerate phosphocreatine stores. In normal muscle, ATP that is broken down to ADP is immediately rephosphorylated to ATP. Thus, phosphocreatine serves as a reservoir of ATP-synthesizing potential. phosphocreatine is the only fuel available to precipitously regenerate ATP during episodes of rapid fluctuations in demand. The availability of phosphocreatine likely limits muscle performance during brief, high-power exercise, i.e., maximal exercise of short duration. With near maximal isometric contraction, the rate of utilization of phosphocreatine declines after 1-2 seconds of contraction, prior to the glycolysis peak at approximately 3 seconds. :45-50.).