In this quantitative neurochemical study we investigated age-related changes in the GABAergic, glutamatergic, and cholinergic neurotransmitter systems in rats and rhesus monkeys. Sixteen young (5 months) and 20 aged (24 months) rats and seven "young" (4-9 years), six "adult" (20-25 years), and five "aged" (29-34 years) monkeys were studied. NMDA-displaceable 1-[3H]glutamate binding was significantly decreased in many neocortical and subcortical regions examined in aged rats and monkeys. The level of choline acetyltransferase (ChAT) activity and [3H]muscimol binding were unchanged in aged animals.