An important indication of the role of brain-derived oestradiol in neuroprotection was provided by the obser¬vation that, in rodents and birds, the brain responds to an acute injury by increasing the expression and activity of aromatase. This was first observed in male and female rodents after an excitotoxic injury in the hippocampus and after stab wound injuries in the cerebral cortex, the hippocampus and other brain regions14. Both types of brain injury resulted in increased aromatase activity in the injured tissue and de novo expression of aromatase in astrocytes, which do not constitutively express the enzyme in adult rodents. These observations were confirmed in birds, which were shown to express aro¬matase in radial glia and astrocytes after brain injury15, and were also extended to other forms of acute brain pathology in rodents, such as experimental stroke16, global ischaemia-reperfusion17 and intracranial pressure raising18. In addition, aromatase expression has been detected in a sexually dimorphic pattern in endothelial cells after brain injury19.