The effects of the co-agonist of the N-methyl-d-aspartate receptor (NMDAr) d-serine on glutamatergic neurotransmission and synaptic potentiation were studied in the CA1 hippocampal field of young (3–5 months old) and aged (25–27 months old) Sprague–Dawley rats using ex vivo extracellular electrophysiological recording techniques. Exogenous d-serine depressed fast neurotransmission mediated by the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate subtype of glutamate receptors in young but not in aged rats by acting on inhibitory glycinergic interneurons. In contrast, d-serine dose-dependently enhanced NMDAr-mediated synaptic responses in both groups of animals, but with a larger magnitude in aged rats, thus preventing the age-related decrease in NMDAr activation. d-serine also increased the magnitude of long-term potentiation in aged but not in young rats. Finally, d-serine levels were dramatically reduced in hippocampal tissues of aged rats. Taken together, these results indicate a weaker activation of the NMDAr glycine modulatory site by endogenous d-serine in aged animals, which accounts for a reduced NMDAr contribution to synaptic plasticity in ageing.