The arginase tissue distribution, the biochemical properties of the argininolytic system and the subcellular localization of the enzymes carbamoylphosphate synthetase, ornithinecarbamoyl transferase, glutamine synthetase and arginase in Antarctic fish, N. neglecta and N. rossii were the main aims of the present work. The tissue with highest argininolytic activity was the kidney distal portion amounting as much as four times the specific activity of the hepatic tissue. Arginase and ornithine carbamoyltransferase were found as mitochondrial enzymes, while glutamine synthetase and carbamoylphosphate synthetase were found as cytosolic enzymes. Argininolytic assays with isolated mitochondria gave values of Kmapp for the hydrolysis of arginine 2 to 3.5 times higher than the values found for the Km with mitochondrial extracts. The effect of Mn2+ on the argininolytic activity displayed by isolated mitochondria and mitochondrial extracts, in reaction conditions near the physiological ones showed that membranes were fundamentally involved in the control of L-arginine metabolism.