Arginase (amidinohydrolase, EC 3.5.3.1) is present in all living organisms, i.e. bacteria, yeasts, plants, invertebrates, and vertebrates. In ureolitic organisms, arginase expresses the highest activity in the liver, where it takes part in ammonia detoxifi cation. Arginase activity is much lower in extrahepatic tissues and its physiological function is still poorly understood; however, it seems to be involved in L-arginine metabolism. Arginase is a homotrimer consisting of 20- to 40-kDa subunits acting at a pH of 10 and in the presence of manganese ions. Proline, ornithine, and NG-hydroxy-L-arginine, an intermediate in the biosynthesis of NO, are known as competitive arginase inhibitors. Two arginase isoenzymes, AI (the so-called "hepatic arginase") and AII ("extrahepatic arginase") are present in mammalian tissues. There are signifi cant differences between the isoenzymes regarding their subcellular localization, isoelectric point, substrate affinity, and immunological cross-reactivity. Arginase isoenzymes AI and AII have high substrate specifi city, but the affi nity to L-arginine is higher for isoenzyme AI than AII. Both isoenzymes exist in most tissues and their expressions change depending on the functional state and metabosynlic requirements. Besides differences in the amino-acid content of the arginase isoforms within one or different species, they have highly conserved regions responsible for the structure and catalytic properties of arginase.