Studies in female rodents have shown that decreas¬ing oestradiol levels in plasma by ovariectomy or other means enhances brain damage under neurodegenerative conditions6–9 and decreases brain glucose metabolism and increases amyloid β oligomers in a mouse model of Alzheimer’s disease10. In turn, the replacement of oestra¬diol levels in the plasma of ovariectomized animals by hormonal therapy reduces damage after brain injury6 and normalizes brain glucose metabolism and decreases amyloid β oligomers in Alzheimer’s disease model mice10. Furthermore, the magnitude of brain damage after an acute neurotoxic or neurodegenerative insult is influ¬enced by the changes in oestradiol levels in plasma dur¬ing the oestrous cycle. For example, the administration of the neurotoxic amino acid kainic acid in the morning of oestrus, after the peak of oestradiol in plasma in the afternoon of pro-oestrus, results in less neuronal damage in the hippocampus than the administration of the toxin in the morning of pro-oestrus, before the oestradiol peak6.