The results of studies on the influence of age on concentrations of vasopressin (VP) in blood plasma and hypothalamic and extrahypothalamic brain sites have not been unequivocal. Studies on extrahypothalamic concentrations of VP in the aging rat have used two age groups only and have mainly provided semiquantitative data. Forthese reasons we determined, by radioimmunoassay, the concentrations of vasopressin in thirteen brain structures and in the plasma of 3-, 10-, 20- and 28-month-old male Wistar rats. Age-related decreases in VP concentrations were found in the pituitary gland, hypothalamus, thalamus, midbrain, medulla oblongata, amygdala and pineal gland, while an increase was noted in plasma. Decreases in the concentration of VP in the amygdala and pineal gland occurred between 3 and 10 months of age and probably represent developmental changes. In the pituitary, thalamus, midbrain, medulla oblongata and plasma, differences in the concentration of VP were also found between 10-month-old and older animals and are probably related to aging. The finding of increased plasma VP concentrations in aged animals agrees with the notion that neuronal function does not necessarily decline with age and suggests that neurons may even be activated. Age-related changes in VP concentrations were not observed in the other structures examined. It has been reported that the VP innervation of a number of brain structures depends on testosterone. Despite reports to the contrary VP concentrations do not generally decline in these structures with aging.