Currently, considerable research activities are focussing on biochemical, physiological and pathological aspects of the creatine kinase (CK) - phosphorylcreatine (PCr) - creatine (Cr) system (for review see [1.2]), but only little effort is directed towards a thorough investigation of Cr metabolism as a whole. However, a detailed knowledge of Cr metabolism is essential for a deeper understanding of bioenergetics in general and, for example, of the effectsof muscular dystrophies, atrophies, CK deficiencies (e.g. in transgenic animals) or Cr analogues on the energy metabolism of the tissues involved. Therefore, the present article provides a short overview on the reactions and enzymes involved in Cr biosynthesis and degradation, on the organization and regulation of Cr metabolism within the body, as well as on the metabolic consequences of 3-guanidlnopropionate (GPA) feeding which is known to induce a Cr deficiency in muscle. In addition, the phenotype of muscles depleted of Cr and PCr by GPA feeding is putinto context with recent investigations on the muscle phenotype of 'gene knockout' mice deficient in the cytosolic muscle-type M-CK. (Mol Cell Biochem 133/134: 51-66. 1994)