A hyperargininemia was induced by oraly administration of arginine in rabbit, and amino acids and guanidino compounds were analysed in the serum, urine and brain. As the experimental results, the following changes in the content of amino acids and guanidino compounds were observed: 1) Urea and ornithine increased, and otherwise, threonine, serine, isoleucine and lysine decreased significantly in the serum of the experimental hyperargininemic rabbit. 2) Among the serum guanidino compounds, only guanidinosuccinic acid increased significantly in the hyperargininemics. 3) Urinaly urea, alanine, cystathionine, ornithine and lysine increased significantly in the hyperargininemics. 4) Taurocyamine, guanidinosuccinic acid, glycocyamine, N-acetylarginie and γ-guanidinobutyric acid increased significantly in the urine of the hyperargininemics, and otherwise several unknown guanidino compounds were observed as higher peaks in the analysing record of the hyperargininemics. 5) Urea and ornithine increased and lysine and l-methylhistidine decreased significantly in the brain of the hyperargininemics. 6) Glycocyamine and N-acethylarginine increased significantly in the brain of the hyperargininemics, but no significant change was observed in other guanidino compounds in the brain of our experimental animal.