Individuals with Down syndrome (DS) develop the pathological hallmarks of Alzheimer's disease at an early age, later followed by memory decline and dementia. Women with DS are twice as likely to undergo early menopause, and levels of estradiol correlate with onset of cognitive decline in these women. We have demonstrated that a mouse model of DS, mice with segmental trisomy of chromosome 16 (Ts65Dn), develop a significant deficit in both reference and working memory as young adults (6-10 months of age), coupled with phenotypic loss of cholinergic neurons in the basal forebrain and altered growth factor levels. In the present study we examined cholinergic and dendritic markers in the hippocampal formation and levels of the amyloid precursor protein (APP) in different brain regions of Ts65Dn mice treated with estradiol for 60 days. The density of the dendritic marker Map2 was significantly decreased in the hippocampal formation of middle-aged trisomic mice (9-15 months old), and the density of cholinergic neurites (acetylcholinesterase [AChE] histochemistry) was also decreased in specific layers of the hippocampus. Treatment with 17beta-estradiol alleviated the decreases in Map2 and AChE staining, but had no effect on full-length APP levels in the hippocampus. In contrast, a main effect of treatment on APP levels in the striatum was noted, with significant elevations observed in controls and trisomics. These findings demonstrate that estrogen can alleviate deficits in cholinergic and dendritic elements in the hippocampal formation and further strengthens the rationale to explore estrogen replacement therapy in women with DS.