The expression of estrogen receptor (ER)alpha and -beta in the infundibular nucleus of the hypothalamus was studied immunocytochemically in 28 control subjects and 14 patients with Alzheimer's disease (AD). A shift was found from more nuclear staining of ERalpha in young female controls to more cytoplasmic staining in elderly female controls, whereas no such change was observed in elderly male controls. The shift of ERalpha from nucleus to cytoplasm in elderly female controls was accompanied by a relative absence of AD neuropathology, i.e. hyperphosphorylated tau stained by hyperphosphorylated tau protein (AT8). In contrast, male and female AD patients showed more nuclear ERalpha and a much stronger AD neuropathology. It is proposed that the shift of ERalpha from nucleus to the cytoplasm may reflect activation of neurons and that hyperactivity decreases the risk that neurons in the course of aging develop AD neuropathology. In contrast, the presence of nuclear ERalpha seems to predispose to reduced activity and increases the risk of some neurons to develop AD neuropathology. ERbeta in basket-like terminals was preferentially observed in elderly male controls and AD patients, a novel phenomenon. This suggests that the presence of basket-like ERbeta may reflect reduced activity, which is-associated with an increase in hyperphosphorylated tau staining. However, the neurons inside the basket-like ERbeta showed signs of hyperactivity and did not stain for AT8. All AT8-positive neurons in the infundibular nucleus contained alphaMSH as a marker for proopiomelanocortin neurons. These neurons produce beta-endorphin that inhibits GnRH release. Because they diminish in activity in postmenopausal women, this may contribute to the hyperactivity of GnRH neurons. The regulation of the gonadal axis may thus be affected by AD neuropathology independent of AD neuropathology in cognition-related brain structures.