Male rats were sacrificed at 5 months (young rats) or at 24 months (aged rats). When compared with values in young rats, aged rats had higher serum concentrations of prolactin and lower concentrations of luteinizing hormone and testosterone. In the median eminence, which contains the terminals of tuberoinfundibular dopamine (DA) neurons, the concentrations of DA and dihydroxyphenylactic acid (DOPAC), and the rate of DA synthesis (accumulation of DOPA after the inhibition of DOPA decarboxylase) were decreased in aged rats. In the striatum, which contains the terminals of nigrostriatal DA neurons, the concentration of DA was reduced, but this change was not accompanied by a decrease in DOPAC concentrations or DOPA accumulation. The decreased DA concentration observed in the median eminence and striatum of aged rats may reflect the loss of DA neurons. An attempt was made to mimic the age-related loss of nigrostriatal and tuberoinfundibular DA neurons by pretreating rats with intraventricular injections of 6-hydroxydopamine. In these animals the decrease of DA in the median eminence was accompanied by a concomitant reduction in the rate of DOPA accumulation, whereas in the striatum the concentration of DA was reduced, but DOPA accumulation remained normal. These results suggest that in aged and 6-hydroxydopamine-treated rats, the loss of nigrostriatal DA neurons is accommodated for by a compensatory increase in the activity of the remaining neurons whereas tuberoinfundibular DA neurons are unable to compensate in a similar manner.