Changes in NMDA receptors in the prefrontal/frontal cortex and hippocampus of C57BL/6 mice during aging show a relationship to declines in spatial learning. The present study was designed to determine whether aging influences the mRNA expression of different splice forms of the zeta1 subunit of the NMDA receptor. We examined the mRNA of 4 C-terminal splice forms with the use of in situ hybridization. The zeta1-1 splice form (+C1 and +C2 cassettes) overall showed a maintenance of mRNA density from 3 to 10 months of age, followed by a significant decline by 26 months of age. In contrast, the mRNA for the zeta1-3 splice form [+C1 and +C2'(-C2)] showed significant declines between 3- and 10-month-old mice. These declines were maintained in the old mice. The zeta1-2 splice form (-C1 and +C2) showed a near-significant decrease in expression during aging across all brain regions. The zeta1-4 subunit mRNA [-C1 and +C2' (-C2)] showed no significant changes with increased age. These results indicate that there is a differential effect of aging on different splice variants of the zeta1 subunit of the NMDA receptor and those that are affected show a different temporal pattern of aging. This heterogeneity has implications for producing imbalances in the modulation of the remaining receptors.