The effect of intraperitoneal administration of scopolamine (1 mg/kg) on acetylcholine (ACh) release in vivo in 3- and 24-month-old freely behaving rats was investigated in the cerebral cortex, hippocampus and striatum by means of transverse microdialysis. In the parietal cortex, the increase in ACh release after scopolamine administration was significantly greater in the old than in the young rats, reaching a maximum increase of about 600 and 300% in the old and young animals, respectively. In the hippocampus, scopolamine caused a larger increase in ACh release in the young (+900%) than in the old rats (+600%). In the striatum of aged rats, a 40% increase occurred only at 40 min after scopolamine administration while in the striatum of young animals the increase lasted for at least 2 h, reaching a maximum of about 100%. These findings demonstrate that the modulation of ACh release in vivo is affected in a different manner in the cerebral cortex than in the hippocampus and striatum by aging.