The degeneration of the basal forebrain cholinergic system plays an important role in cognitive deterioration in aging and Alzheimer's disease. Brain cholinergic neurons and their projections are affected by changes in the circulating levels of estrogens, which exert their effects mainly through the estrogen receptors. In this study, we investigated the effect of aging, estrogen status and transgenic genotype on the number of cholinergic neurons and the estrogen receptor alpha (ERalpha) content in the medial septum-vertical limb of the diagonal band of Broca. We used 6- and 12-month-old female double transgenic mice carrying mutated human amyloid precursor protein (APPswe) and presenilin-1 (PS1-A246E), and their nontransgenic littermate controls, which had been sham-operated or ovariectomized at the age of 3 months. Brain sections were double immunostained for choline acetyltransferase (ChAT) and ERalpha and used for stereological cell counting. We found that the number of ChAT-immunoreactive (ir) neurons containing nuclear ERalpha-ir was significantly lower in 12- than in 6-month-old mice. However, the age of the mice, the transgenic genotype or ovariectomy had no effect on the total number of ChAT-ir neurons, or on the number and percentage of all ChAT-ir neurons that contained ERalpha. These results indicate that aging is associated with translocation of ERalphas from the nucleus to the cytoplasm. We propose that this phenomenon is linked to those age-related processes known to be involved in inhibiting ERalpha binding to nuclei.