There is evidence that physical activity is associated with decreased brain injury resulting from transient middle cerebral artery (MCA) occlusion. We investigated whether exercise could reduce stroke-induced brain inflammatory injury and its associated mediators. Sprague Dawley rats (3 months old) were subjected to 30 min exercise on a treadmill each day for 1-3 weeks. Stroke, in exercised and non-exercised animals, was then induced by a 2-h MCA occlusion followed by 48 h of reperfusion using an intraluminal filament. Endothelial expression of the intercellular adhesion molecule 1 (ICAM-1) and leukocyte infiltration were determined by immunocytochemistry. Expressions of tumor necrosis factor-alpha (TNF-alpha) and ICAM-1 mRNA were detected using a real-time reverse transcriptase-polymerase chain reaction in ischemic rats with or without exercise, and in non-ischemic control rats following exercise. Expression of TNF-alpha increased after exercise for 2 and 3 weeks. The overexpression of TNF-alpha was not further elevated in 3-week exercised rats subjected to a transient MCA occlusion and 6 or 12 h of reperfusion, as compared to that in non-exercised rats. Furthermore, ICAM-1 mRNA expression remained at significantly (P<0.01) low levels in exercised animals during ischemia/reperfusion. Pre-ischemic exercise significantly (P<0.01) reduced numbers of ICAM-1-positive vessels and infiltrating leukocytes in the frontoparietal cortex and dorsolateral striatum in ischemic rats after 48 h of reperfusion. Exercised ischemic rats demonstrated an 11+/-7% infarct volume of contralateral hemisphere as compared to a 52+/-3% volume in non-exercised ischemic rats. The data suggests that exercise inhibits inflammatory injury (i.e., decreased expression of inflammatory mediators and reduced accumulation of leukocytes) during reperfusion, leading to reduced brain damage. Chronically increased expression of TNF-alpha during exercise prevent the same downstream inflammatory events as does acutely elevated TNF-alpha after ischemia/reperfusion.