The fine structural features and water content of white matter associated with the resolution process of brain edema were sequentially investigated in the model produced by infusion of autoserum, mock CSF, or ferritin into the centrum semiovale of cats. The correlation between water content and morphological features was good. Mock CSF-infused edema disappeared within 3 days, serum infused edema within 6 days. In a fine structural study of serum-infused white matter, the distended extracellular spaces were found to be occupied with electron-dense materials, active phagocytosis of the dense materials being observed in the macrophages. Around the postcapillary venules, edematous changes were characterized by wide expansion of the perivascular spaces between endothelial cells and astrocytic endfeet. In some instances, the dense materials in the cytoplasm or in the membrane-bound vacuoles of the astrocytic endfeet were continuous with those in the perivascular space, through the hiatuses of the perivascular astrocytic endfeet being separated at their margins. At 3 days after infusion, wide distension of the extracellular space persisted, but the dense materials had markedly diminished. These results strongly suggest that water clearance of vasogenic brain edema does not commence until proteinaceous macromolecules are degraded and removed from the extracellular space. Perivascular channels around the postcapillary venules might also have some role on the movement of edematous fluid.