The effects of the 5-HT agonist meta-chlorophenylpiperazine (MCPP) on regional cerebral metabolic rates for glucose (rCMRglc) were measured in 3- and 24-month-old rats that were not pretreated or were pretreated for 2 weeks with continuous infusion of saline or MCPP. rCMRglc were measured using the quantitative autoradiographic [14C]2-deoxy-D-glucose technique in 71 brain regions at 15 min after acute administration of MCPP 2.5 mg/kg. In the absence of chronic pretreatment, intraperitoneal MCPP 2.5 mg/kg produced widespread rCMRglc reductions (41 brain areas) in 3-month-old rats and more limited rCMRglc decreases (8 brain areas) in 24-month-old rats. After chronic treatment, MCPP failed to reduce rCMRglc in any region of either group of rats. These findings indicate that mechanisms of downregulation of response to MCPP are functional in young and aged rats and suggest that the age-related reduction in rCMRglc responses to acute MCPP in non-pretreated animals may be due to compensation for age-related losses of 5-HT terminals.