Brain water content, brain blood volume, blood chemistry, and pathology in a model of cerebral edema. STUDY OBJECTIVES: The objective was to correlate regional changes during brain water content with alterations in blood chemistry and cerebral pathology during hypo-osmotic edema. PARTICIPANTS: Sprague-Dawley male adult rats were used in these studies. DESIGN: Animals were block-randomized to receive either an intraperitoneal distilled water injection equivalent to 5% or 15% of their body weight or no injection (controls). Rats were sacrificed 15 or 60 minutes after water injection or at an equivalent time for controls. INTERVENTIONS: No interventions were performed. MEASUREMENTS AND MAIN RESULTS: Water content of cerebral cortical gray and white matter was calculated from measurements of tissue specific gravity. Blood plasma osmolality and sodium and potassium concentrations were determined at various times after water injection. An index of blood-brain barrier permeability was obtained by measuring brain red blood cell and plasma volumes. A qualitative assessment of edema was made from light and electron micrographs of the cerebral cortex. We found that water injection produced a dose-dependent decrease in plasma osmolality and sodium concentration within 15 minutes. Cortical water content was unchanged after this period. An influx of water into cerebral gray, and, less readily, into cerebral white matter occurred during the next 15 minutes. Whole blood specific gravity and brain blood content were unchanged and thus did not confound the measurement of cerebral water content. Hematocrit was increased 60 minutes after a 15% water injection. The blood-brain barrier remained intact throughout this period. Microscopy revealed astrocytic swelling with slight extracellular fluid accumulation 60 minutes after the water injection. CONCLUSIONS: Homeostatic mechanisms in the cerebral cortex can maintain constant water content for at least 15 minutes during maintained intravascular hypo-osmolality. Fluid that subsequently moves into the tissue primarily enters an intracellular compartment. This model will be useful in investigating physiological mechanisms of brain water regulation and the pathogenesis of brain edema, a common clinical entity in emergency conditions. 