The present study evaluates the effects of agmatine on histopathological damage following traumatic injury using a clinically relevant model of diffuse brain injury. A total of 27 male Sprague-Dawley rats weighing 200-225 g were anaesthetised and subjected to head trauma using Marmarou's impact-acceleration model. The rats were then separated into two main groups: one was treated with agmatine and the other with saline for up to 4 days immediately after head trauma. Rats from both groups were killed 1, 3 or 8 days post-injury. The brains were examined histopathologically and scored according to the axonal, neuronal and vascular changes associated with diffuse brain injury. There were no significant differences between the groups at 1 day or 3 days after trauma, but evaluation after 8 days revealed a significant improvement in the group treated with agmatine. Our data indicate that agmatine has a beneficial effect in diffuse brain injury and should be trialled for therapeutic use in the management of this condition.