The evolution of arginase activity during development of Bufo arenarum embryos was studied. The enzyme activity was detected after the neural groove stage, being low at the following early stages and increasing progressively until the end of the embryonic development. Arginase activity was first found in the neurula stage, when an outline of basic structures for renal and hepatic function is present. The activity increased two- or three-fold at the beginning of the larval stages.