Recent findings indicate that oestradiol synthesis in the hippocampus is regulated by gonadotropin-releasing hormone162, which may coordinate oestradiol synthesis in the brain and the ovary. Furthermore, aromatase activity, and therefore oestradiol production, is rapidly regulated in the brain by glutamatergic synapses155. The synaptic location of aromatase, together with the rapid regulation of its activity by neurotransmitters, suggests that brain-derived oestradiol acts as a rapid neuromodulator in synaptic circuits155,163. Indeed, rapid changes in brain aromatase activity are linked to swift changes in synaptic function, neuronal activity and behaviour155,158. Thus, local oestradiol synthesis in the brain, modulated by rapid changes in aromatase activity, seems to play an important part in the regulation of brain function and behaviour under physiological conditions.