To investigate the contribution of cytokines, proinflammatory TNF-alpha and inhibitory TGF-beta, to spinal cord injury (SCI) in a rat model, two studies were performed using adult Sprague-Dawley rats which were injured at T9/T10. In the first study, rats were sacrificed at 1, 6, 24, 96 and 168 h after SCI for immunocytochemistry of coronal sections for the presence of mononuclear phagocytes, astrocytes, TNF-alpha and TGF-beta, among other markers. From intervening frozen sections, RNA was extracted for semiquantitative polymerase chain reaction (RT-PCR) analysis of TNF-alpha and TGF-beta. In the second experiment, rats were treated with intravenous TGF-beta 30 min after injury and sacrificed at 6 and 48 h after injury. Spinal cord sections were immunocytochemically stained and RNA extracted for semiquantitative PCR as mentioned above, as well as quantitation of lesion volume. There were increases in mononuclear phagocytes and astrocytes, as early as 1 h after SCI, with steady progression over 168 h after injury. TNF-alpha and TGF-beta was produced locally by mononuclear phagocytes and astrocytes. There was an 18-h delay in peak mRNA production of TGF-beta compared to TNF-alpha. The treatment of SCI rats with TGF-beta reduced lesion volume by 50% at 48 h and this was associated with decreased accumulation of mononuclear phagocytes in and around the injury site. This reduction of mononuclear phagocyte numbers around the site of trauma would reduce their contribution to secondary injury.