Cerebrovascular permeability to 14C-sucrose was measured independently of cerebral blood flow in the rat, following unilateral osmotic opening of the blood-brain barrier by hypertonic arabinose solution. An increase in regional permeability to 14C-sucrose was correlated with the extent of brain staining by intravascular Evans blue-albumin, a visual barrier tracer. To obtain the product of capillary permeability and surface area, 14C-sucrose was injected intravenously, the arterial plasma concentration curve was determined and integrated over a 10-minute period, and brain parenchymal concentration was calculated after subtracting vascular radioactivity from net tissue radioactivity. Conditions were chosen so that a simple diffusion equation could be applied to exchange between plasma and brain compartments, in the absence of back flux from brain. The method should be of use for studying effects of microwaves on the cerebral vasculature.