Diffusion of solutes and macromolecules in brain extracellular space (ECS) is important for normal brain function and efficient drug delivery, and is thought to be impaired in edematous brain. Here we measured the diffusion of an inert macromolecular fluorescent marker (FITC-dextran, 70 kDa) in the ECS by fluorescence recovery after photobleaching after staining the exposed cerebral cortex in vivo. In a brain tumor model of vasogenic (leaky capillary) edema, FITC-dextran diffusion was reduced more than fourfold in hypercellular tumor and surrounding astrogliotic tissue; however, diffusion in brain away from the tumor was approximately 30% faster than in normal contralateral brain. The increased diffusion was abolished by dexamethasone pretreatment. Enhanced ECS diffusion was also found in uninjured brain near a region of leaky brain vessels produced by focal cortical freeze injury. In contrast, ECS diffusion was slowed more than sixfold in cytotoxic brain edema caused by anoxia. Diffusion results were related semiquantitatively to ECS volume fraction and matrix viscosity from in vitro photobleaching studies in a model system consisting of silica particles in a fluorescent water/glycerol matrix. Our data provide in vivo evidence for enhanced ECS diffusion in vasogenic brain edema, yet greatly slowed diffusion in cytotoxic edema and in and around tumors.