(1) Rodent chronically treated with d-galactose (d-gal) is increasingly used in pharmacological studies on aging; however, its mechanism remains unclear. The present study investigated the alterations of gene expression in the hippocampus of d-gal-treated mice. (2) C57 mice were subcutaneously injected with d-gal for 2, 4, and 8 weeks or vehicle, and then were subjected to behavioral tests. Gene expression profiles in hippocampus of each group were finally examined with cDNA microarray. (3) Both 4- and 8-week d-gal treatment led to a decrease of discrimination index in object recognition test, and 8-week d-gal-treated mice showed significant spatial learning & memory impairment in Morris water maze test. In comparison with the vehicle control group, the 2-, 4-, and 8-week d-gal treatment repressed the expression of 10, 13, and 30 genes by 2-fold or more, respectively. The early pattern was mainly characterized by down regulation of genes involved in ion transport. The delayed pattern included genes involved in protein biosynthesis, transport and signal transduction, which were highly related to synaptic functions. (4) These findings will contribute to the understanding of the mechanism of learning and memory impairment in mice treated with d-galactose. © 2007 Springer Science+Business Media, LLC.