The cerebral Aquaporin-4 (AQP4) water channel is suggested to be involved in brain edema formation aggravated by reduced cerebral blood flow early after traumatic brain injury (TBI). Therefore, the temporal profile of brain edema formation, AQP4 expression, and cortical perfusion were investigated following focal TBI in rats. Brain edema was maximal by 24 h. Concurrently, AQP4 protein expression was decreased in both hemispheres, being more pronounced in the traumatized hemisphere (-50%) 48 h after trauma. Cortical perfusion was only decreased in the ipsilateral cortex (-40%) between 4 and 8 h after trauma, reaching baseline values at 24 h. Globally reduced AQP4 expression following induction of a focal contusion coincides with edema development and seems to be independent of changes in cortical perfusion.