A gamma-guanidobutyrate ureahydrolase isolated from tench liver has been characterized. Some of its physicochemical properties like pH effect and thermal stability resemble those of arginases, however it shows some peculiarities that makes it different from arginases and other amidino hydrolases. Thus cation requirement is not as strong as in arginases, and the Km value for gamma-guanido-butyric acid (230 +/- 25 mM) is shifted to a lower value (45 +/- 5 mM) by 5 mM arginine. The possible regulatory role of arginine on gamma-guanidobutyrate ureahydrolase activity is discussed.