The site of the initial cleavage in the metabolism of oxytocin and arginine—vasopressin was studied in the isolated perfused rat kidney. When kidneys were perfused with either of these hormones, which were labeled with 14C in the glycine residue, the only radioactive substances detected in the urine were the intact hormone and one metabolite, glycinamide. When 3 μg of oxytocin or vasopressin was added to the perfusate, the urinary glycinamide amounted to 85% or 60%, respectively, of the total radioactivity excreted. When the perfusion medium contained initially 10 μg of oxytocin, the recovered radioactivity in the urine was present exclusively in the form of glycinamide. (Endocrinology92: 189, 1973)