This study was designed to determine if changes related to aging and diet in the mRNA expression of subunits of the NMDA receptor were associated with changes in binding to NMDA receptors and learning ability in C57Bl/6 mice. Three age groups (3, 15, and 26-27 months old) and 2 diet groups (ad libitum-fed and diet restricted) were used. The old ad libitum-fed mice had significantly poorer performance in a spatial reference memory task than all other groups. Diet restriction slightly spared glutamate binding to NMDA sites and improved zeta1, but not epsilon2, mRNA expression. Significant correlations were found between NMDA-displaceable [(3)H]glutamate binding and both learning ability and epsilon2 and epsilon1 mRNA density in several brain regions. Learning ability in the old mice also correlated with the ratios of mRNA expression for epsilon1 and epsilon2 and/or zeta1 subunits in the parietal cortex and CA1 region of the hippocampus. This suggests that it is the relationship between subunit expression levels that is important for maintaining memory functions in older animals.