Under normal conditions a slight vesicular transfer of intravenously injected horseradish peroxidase (HRP) occurs across the endothelium of cerebral vessels, especially short segments of arterioles. The vesicular transport can be notably increased by chemically induced acute hypertension. In the present investigation 4 groups of animals received HRP, and the permeability of the cerebral endothelium was studied semimacroscopically, light microscopically and electron microscopically. The rats in group 1 were given 10 electroshocks. This caused a significant rise in the blood pressure (BP). Furthermore, a noticeable extravasation of HRP was observed, especially across the endothelium in cerebral arterioles. From the basement membranes of the vessels reaction product could be followed into the extracellular spaces of the neighbouring neuropil. Group 2 comprises rats that were given 10 electroshocks preceded by transection of the cervical part of the spinal cord. The BP remained at normal level and the permeability was unaltered. The animals in group 3 received only 1 electroshock. Usually, the BP was markedly increased and this was accompanied by enhanced permeability across the vessels of the brain. Group 4 consists of control animals, injected with HRP and treated as groups 1 and 3 with the difference that electrical stimulation was not performed. A general feature was that no endothelial damage was observed and that reaction product was not found between neighbouring endothelial cells from the first luminal to the first abluminal tight junction. Based on the observations it seems reasonable to assume that the increased permeability of tracer that occurs after 10 electroshocks or only one is caused by the acute hypertension evoked by the electrical stimulation; furthermore, the transfer is concluded to be vesicular transport.