We assessed baseline and KCl-stimulated glutamate release by using microdialysis in freely moving young adult (7 months) and middle-aged (17 months) transgenic mice carrying mutated human amyloid precursor protein and presenilin genes (APdE9 mice) and their wild-type littermates. In addition, we assessed the age-related development of amyloid pathology and spatial memory impaired in the water maze and changes in glutamate transporters. APdE9 mice showed gradual spatial memory impairment between 6 and 15 months of age. The stimulated glutamate release declined very robustly in 17-month-old APdE9 mice as compared to 7-month-old APdE9 mice. This age-dependent decrease in stimulated glutamate release was also evident in wild-type mice, although it was not as robust as in APdE9 mice. When compared to individual baselines, all aged wild-type mice showed 25% or greater increase in glutamate release upon KCl stimulation, but none of the aged APdE9 mice. There was an age-dependent decline in VGLUT1 levels, but not in the levels of VGLUT2, GLT-1 or synaptophysin. Astrocyte activation as measured by glial acidic fibrillary protein was increased in middle-aged APdE9 mice. Blunted pre-synaptic glutamate response may contribute to memory deficit in middle-aged APdE9 mice.