Guinea pig neurohypophysial hormones have been purified by two procedures, one involving molecular sieving and paper chromatoelectrophoresis, the other high-pressure reverse-phase liquid chromatography. Arginine vasopressin and oxytocin have been identified by their amino acid compositions and their retention times in HPLC determined through their biological properties. No partially processed precursor, including a neurohormone and a neurophysin, has been detected. Because the cleavage of the three-domain vasopressin-neurophysin-copeptin precursor is apparently complete between the first two domains, whereas it is not between the second and the third, it is supposed that two distinct enzymic systems are involved in the processing.