Conclusion: AGAT and GAMT knockouts express a similar neuroanatomical phenotype that, interestingly and in contrast to patients, is more pronounced in AGAT deficiency. Phosphocreatine serves as an important reservoir of high-energy phosphate for ATP synthesis in tissues that have fluctuating demands for energy. Human disorders of creatine biosynthesis and transport exist, leading to intellectual disabilities, epilepsy, and poor growth. Creatine biosynthesis requires two enzymes, the first, L-arginine:glycine amidinotransferase, the product of the Gatm gene, catalyzes the rate-limiting transfer of the amidino group from arginine to glycine, yielding guanidinoacetate and ornithine. We generated a mouse strain lacking Gatm. Homozygous Gatm deficient mice have an almost complete absence of creatine and guanidinoacetate, and exhibit poor growth and muscle weakness. There is a reduced amount of glycogen in the liver in conjunction with reduced glycogen synthase activity, lipid accumulation in liver, skeletal and cardiac muscle, and a marked hypoplasia of skeletal muscle. A consequence of creatine deficiency is the global reduction in skeletal muscle mitochondrial respiratory chain activities, and a compensatory increase in mtDNA content and citrate synthase activity. Muscle mitochondria accumulate striking paracrystalline arrays of cristae. Cardiac function as assessed by stress echocardiography is normal. Treatment with creatine leads to muscle proliferation and improves strength. This animal model of creatine deficiency demonstrates the central role creatine metabolism plays in growth and muscle homeostasis. Background: The creatine/creatine-phosphate system plays an essential role in keeping energy homeostasis in human tissues with high energy demand, such as brain and muscle. Deficiencies of creatine can cause developmental delay, intellectual disability and behavioral disorders. Three genes are involved in inborn errors of creatine deficiency syndromes: autosomal recessive GAMT and AGAT, as well as an X-linked SLC6A8 (CT1) encoding a creatine transporter. Deficiency of these enzymes can be distinguished by the concentrations of guanidinoacetate (GAA), creatine, and creatine/creatinine ratio in plasma and urine. However, biochemical analyses do not provide definitive diagnosis.