A quantitative autoradiographic technique that utilizes carbon-14-aminoisobutyric acid (14C-AIB) as a tracer was used to study alterations in cerebral microvascular permeability in 15 rats. Five were "sham-operated" controls and 10 underwent microsurgical, unilateral occlusion of the proximal middle cerebral artery (MCA). Histological changes indicative of focal cerebral ischemia were observed in only the latter 10 animals. These changes were confined to tissue normally perfused by the occluded MCA. After MCA occlusion, five animals were also subjected to transient halothane-induced hypotension (mean arterial blood pressure 50 mm Hg) for 20 to 30 minutes. Only in these five animals were blood-to-brain transfer constants (ki) significantly increased (by approximately 100%) at 4 hours after MCA occlusion. The topographical distribution of this alteration in cerebral microvascular permeability corresponded closely with the histological changes. Neither proximal MCA occlusion nor halothane-induced hypotension alone was associated with any focal or diffuse increase in ki after 4 hours.