Background We have pursued the concept that traumatic brain edema is predominantly cellular and that water entry is modulated in part by aquaporins. Aquaporin-4 (AQP4) has been shown to play a significant role in cellular edema formation. Phorbol myristate acetate (PMA) is a potent PKC activator; purportedly involved in modulation of AQP4 activity. Alternatively, AQP4 may be regulated by arginine-vasopressin. Administration of the vasopressin antagonist (SR49059) reduced brain water content and sodium shift following MCAo. To investigate if edemaformation is affected by the reduction of AQP4 expression, we utilized PMA and SR49059 following middle cerebral artery occlusion model (MCAo), and measured AQP4 expression by Western-Blot (WB) techniques. Methods Male Sprague Dawley rats were randomly assigned to sham (n=4) or MCAo groups (vehicle, PMA or SR49059 infusion; n=6 each). Each solution was infused for 5 hours, starting 1 hour before injury. After a two-hour period of ischemia and two-hour reperfusion, animals were sacrificed and brain regions of interest were processed by WB to quantify the effect of treatment on AQP4 expression. Results These studies demonstrate that MCAo results in a significant up-regulation of AQP4 on the ischemic zone when compared to the contralateral un-injured hemisphere (p<0.05) and that PMA and SR49059 treatment significantly down-regulated AQP4 expression compared to the vehicle group (p<0.05). Conclusions These studies support the hypotheses that PMA and SR49059 may be useful in reducing cerebral water accumulation by modulating AQP4 expression and that pharmacological manipulation of AQP4 may emerge as a viable strategy for the reduction of fulminating edema following ischemic injury. © 2008 Springer-Verlag/Wien.