Focal vasogenic brain edema was induced by ultraviolet irradiation of the exposed cerebral cortex of 16 cats. In 5 animals horseradish peroxidase was intravenously injected at times varying from 30 min to 24 h following irradiation and allowed to circulate for 45 min. Fixation was carried out by perfusion and immersion with glutaraldehyde. The tissue, part of it incubated for peroxidatic activity, was treated for electron microscopy. The UV-irradiation leads to a shallow coagulation of the superficial cortex from which a wide zone of edematous tissue spreads to the deep white matter within 24 h. Arterioles, capillaries and venules of this zone show enhanced pinocytotic activity and a concurrent rise in permeability for horseradish peroxidase which is found in micro- and macropinocytotic vesicles, in tubular vesicle-like structures, in endothelial wall invaginations and apparently not membrane-bound. These changes are most pronounced in venules which after 48 h allow penetration of reaction product though the base membrane into the surrounding neuropil. There is no evidence for the penetration of tight junctions which appear intact.