The autoregulatory feedback control of prolactin, which is mediated by tuberoinfundibular dopamine (DA) neurons, is altered in the aged rat; this is evidenced by increased circulating concentrations of prolactin and decreased activity of these neurons. In the present study the action of prolactin on tuberoinfundibular DA neurons in young and aged female rats was estimated by measuring the rate of DA synthesis (dopa accumulation following the administration of a decarboxylase inhibitor) in the median eminence. The rate of dopa accumulation in the median eminence of the aged (26 months) rat was reduced to 50–60% of that in the young (3 months) rat. The acute systemic administration of haloperidol, a DA antagonist which increases serum concentrations of prolactin or intracerebroventricular infusions of prolactin increased the rate of dopa accumulation in the median eminence of both young and aged rats by the same relative amount. The administration of haloperidol and prolactin increased the rate of DA synthesis to a greater extent in young than in aged rats. The administration of bromocriptine, a DA agonist which reduces serum concentrations of prolactin, decreased the rate of dopa accumulation in the median eminence of both young and aged rats. In young animals the intracerebroventricular administration of prolactin reversed the bromocriptine-induced decrease in DA synthesis in the median eminence after 4 h and caused a further increase after 12 h. Qualitatively similar effects were seen in the aged rats; however, prolactin-treated young rats had much higher levels of DA synthesis than aged rats. Studies were undertaken to determine the response of DA synthesis in the median eminence of young and aged rats to chronically elevated prolactin. In these studies elevated serum prolactin was sustained by eleven daily injections of haloperidol. The rate of dopa accumulation in the median eminence of both young and aged rats was increased by the same relative amount but not to the same absolute levels. Despite the age-related decrease in the basal activity of tuberoinfundibular DA neurons, these neurons in young and aged rats are equally responsive to prolacti