The radioimmunological analysis revealed an increase in the concentration of arginine vasopressin in the cerebrospinal fluid of rats during aging. This correlated with the results of karyometry of neurosecretory neurons of the supraoptic nucleus of the hypothalamus which showed an increase in the size of their nuclei. Under conditions of water deprivation of both adult and old rats, there was an increase in the above parameters being more marked in the adult than old rats. All this indicated pronounced changes of the vasopressin regulation in aging, limited reserves of the hypothalamic-neurohypophysial system, and age-related decrease of its response to stress. An enhanced role of the cerebrospinal fluid in accomplishment of vasopressin-induced regulation of brain functions in the old age was suggested.