Summary. The synthesis of urea from y-guanidinobutyric acid (GGBA) and arginine and the possible distribution of arginase and γ-guanidinoureahydrolase has been investigated. The enzyme splitting GGBA in brain has not been described before. Arginase, as well γ-guanidinoureahydrolase, is fairly thermorésistant. The specific activity of arginase in proteins soluble in 50% acetone is 5.4 times more and of γ-guanidinoureahydrolase 5.5 times more than that in the initial homogenate. The investigation has proved our conception of the existence of the last two ways of urea formation in brain.