Potential age differences in N-methyl-D-aspartate (NMDA) receptor binding were evaluated in rat brain using [ 3 H]dizocilpine and [ 3 H]D,L-(E)-2-amino-4-propyl-5-phosphono-3-pentanoic acid (CGP 39653) as the radioligands. Neither [ 3 H]dizocilpine nor [ 3 H]CGP 39653 binding in hippocampus revealed any age difference (7 vs. 23-25 months) in B m a x or K d . High-affinity [ 3 H]dizocilpine binding showed a developmental increase (8 vs. 2 months) in hippocampus but not in frontal cortex. No senescence-related change (25 vs. 8 months) was observed in either region. The lack of senescence-related differences in these biochemical markers does not preclude the possibility of functional changes in the NMDA receptor with aging.