Although estrogen is best known for its effects on the maturation and differentiation of the primary and secondary sex organs, increasing evidence suggests that its influence extends beyond this system, and its activity in the CNS may initiate, or influence our susceptibility to neurodegenerative decline. Estrogen has been proposed to act as a neuroprotectant at several levels, and it is probable that deprivation of estrogen as a result of menopause exposes the aging or diseased brain to several insults. In addition, estrogen deprivation is likely to initiate or enhance degenerative changes caused by oxidative stress, and to reduce the brain's ability to maintain synaptic connectivity and cholinergic integrity leading to the cognitive decline seen in aged and disease-afflicted individuals.