Reduced number of neurons is a common feature in Down's syndrome (DS) brains. Since reduced neuronal number also occurs in the dentate gyrus of Ts65Dn mice (TS), a model for DS, hippocampal cell proliferation and survival were analyzed in young and old TS mice. For evaluating proliferation and survival, half of the mice were sacrificed 1 day, and the other half 30 days after the last bromodeoxyuridine injection, respectively. No difference was found in the number of proliferating or surviving cells of young TS and control mice. An age-associated decline in total cell number and density has been found in both genotypes, this decline being more pronounced in TS animals. Thus, aged TS mice showed reduced cell proliferation and density of surviving cells compared to CO mice. Due to the putative involvement of newborn cells in the dentate gyrus in learning processes, the reduced proliferative capacity found in TS mice could be involved in the cognitive problems found in this model of Down syndrome.