Abstract Traumatic brain injury (TBI) is a major health problem, representing a third of all injury-related deaths in the United States and 70% of long-term disabilities in survivors. Decades of TBI research focused almost exclusively on neuroprotective strategies, has failed to develop any therapeutics for clinical treatment. One less explored potential target is the cerebral circulation. In TBI, there is increasing recognition that the peri-contusional areas of TBI suffer microvascular failure and diffusional hypoxia and edema. Our studies on microvascular shunts (MVS) with high intracranial pressure (ICP) corroborate microcirculatory failure. We propose here modulation of hemodynamics with blood soluble drag reducing polymers (DRP) as a novel treatment modality for TBI that specifically targets cerebral microcirculation and that based on physical but not pharmacological principles. Nanomolar amounts of intravenous DRP reduce blood pressure loss in arterioles by diminishing flow separations and microvortices at vessel bifurcations, increase precapillary pressure and the density of functioning capillaries. Increased vascular wall shear rate may reduce transcapillary macrophage migration and inflammation. We showed that 140 µg/kg of intravenous DRP (ED70) increased blood flow velocity in cerebral arterioles, reduced MVS, restored perfusion in capillaries and reduced tissue hypoxia in a rat model of TBI when i.v. injected 30 minutes after the insult. The next logical step, our objective, is to perform a comprehensive study of the dose and time-related efficacy of DRP and to examine the therapeutic mechanisms involved. Central hypothesis: DRP, through their general dose-dependent action on cerebrovascular microcirculation, can present a unique and effective therapy for TBI, applicable at both, early and later time. The rationale is that unlike other TBI therapies tested thus far, the hemorheological effects of DRP are independent of tissue status in terms of tissue or vascular receptor reactivity or sensitivity for its mechanism of action. Our long-term goal is to optimize the application of DRP after TBI for maximal efficacy on long-term recovery and provide for the first time, a therapeutic intervention that may be effective even if delayed hours after injury. Using the lateral fluid percussion injury TBI model in rats, we will address two aims: 1) to study the acute dose-dependent effects of DRP on the time course and relative changes in cerebral microvascular flow, i.e. MVS, tissue oxygenation and metabolism using in-vivo 2-photon laser microscopy and laser speckle imaging after moderate and severe TBI; and 2) to define the optimal dose and therapeutic time window of DRP for clinically relevant long-term outcomes and mechanisms involved using magnetic resonance imaging, behavioral testing and histology, possible anti-inflammatory effects of rheological modulation will be evaluated by ELISA and immunohistochemistry. To comply with NIH requiremen...