There is increasing evidence that there is a direct response of the cerebral microvasculature to head injury. We have investigated using SEM and TEM the response of microvessels within the white matter of the baboon brain to lateral head acceleration. There is rapid endothelial disruption and swelling of perivascular astrocytes near the sites of petechial haemorrhage. The formation of microvilli in all vessels reaches a peak at 6 h and extends at least 5 mm from the site of haemorrhage. The astrocyte response suggests a partial recovery by 6 h. The endothelial response is most marked in arterioles and venules and is maintained for 6 days after injury. We suggest there is a biphasic cerebrovascular response to brain injury. First there is rapid astrocytic swelling possibly correlated with transient disruption of the blood-brain barrier. This is followed by morphological changes in the endothelium of all vessels which are most marked in arterioles and venules and extend considerable distances throughout the neuropile. This response is discussed in the light of disruption of the blood-brain barrier.