1. Livers of ureotelic and uricotelic animals possess the capacity, to a varying extent, of hydrolysing l-arginine and other guanidino compounds. 2. Three different enzymes responsible for such activity have been separated and partially characterized. They are the ;ureotelic' arginase, the ;uricotelic' arginase (both of them specific for l-arginine) and the guanidinobutyrate ureohydrolase (able to hydrolyse beta-guanidinopropionate, gamma-guanidinobutyrate and d-arginine but not l-arginine or guanidinoacetate). 3. The implication of the advent of a ;ureotelic' arginase in the integration of the Krebs-Henseleit cycle is discussed.