The authors studied the effect of a mild cortical contusion to the rat brain on behavioural and morphological outcome and the influence of NMDA-receptor blockade (MK-801, 0.5 mg/kg i.v. 30 min prior to trauma). Spontaneous motor activity was assessed 16-18 days post trauma. Saline treated traumatised rats showed a significant (p < 0.01) hyperactive behaviour compared to animals without injury. MK-801 treated rats performed significantly better than the saline treated animals (p < 0.05). For histopathological evaluation hippocampal hilar neurons were counted, cortical thickness under the impact was measured and microtubule-associated protein 2 (MAP2) immunoreactivity in the dentate hilus was quantified 1, 3 and 21 days post trauma. In traumatised rats scattered loss of nerve cells, oedema and minute haemorrhages were present at the site of the impact one and three days after injury. At day 21 there was a significant reduction of cortical thickness at the site of impact. One day after trauma there was a bilateral, significant loss of neurons and MAP2 immunostaining in the dentate hilus of the hippocampus. MK-801 pretreated rats showed similar morphological changes. The disturbed spontaneous motor behaviour may be caused by hippocampal damage and a reduction of somatosensory cortical neurons. NMDA-receptor blockade improved the outcome assessed by the functional tests but failed to influence the morphological changes, suggesting that this behavioural test is a more sensitive indicator of outcome after mild traumatic brain injury (TBI).