Dopamine secreted into the pituitary stalk blood of old constant estrous rats (20-24 months of age) was significantly less than that of young estrous rats (3-4 months of age). Reduced concentrations of dopamine were also observed in the median eminence and in the neurointermediate lobe of the pituitary gland of old female rats compared to those of young female rats. The low rate of secretion of dopamine into pituitary stalk blood of old female rats was associated with high secretion of prolactin into arterial blood. The impaired hypothalamic secretion of dopamine observed in old rats was not affected by increased availability of L-tyrosine. However, when L-DOPA was given to old rats, very high concentrations of dopamine were measured in pituitary stalk blood, whereas the concentrations of dopamine in the arterial blood were very low. On the basis of these data, it is concluded that the neurosecretory activity of the dopaminergic neurons of the hypothalamus is impaired in old constant estrous rats. This impaired activity can be overcome by increasing the availability of L-DOPA but not L-tyrosine.