Increased neuronal glucocorticoid exposure may underlie interindividual variation in cognitive function with aging in rodents and humans. 11 beta-Hydroxysteroid dehydrogenase type 1 (11 beta-HSD1) catalyzes the regeneration of active glucocorticoids within cells (in brain and other tissues), thus amplifying steroid action. We examined whether 11 beta-HSD1 plays a role in the pathogenesis of cognitive deficits associated with aging in male C57BL/6J mice. We show that 11 beta-HSD1 levels increase with age in CA3 hippocampus and parietal cortex, correlating with impaired cognitive performance in the water maze. In contrast, neither circulating corticosterone levels nor tissue corticosteroid receptor expression correlates with cognition. 11 beta-HSD1 elevation appears causal, since aging (18 months) male transgenic mice with forebrain-specific 11 beta-HSD1 overexpression (similar to 50% in hippocampus) exhibit premature age-associated cognitive decline in the absence of altered circulating glucocorticoid levels or other behavioral (affective) deficits. Thus, excess 11 beta-HSD1 in forebrain is a cause of as well as a therapeutic target in memory impairments with aging.