Retrograde infusion of a hypertonic arabinose solution into the right external carotid artery of rats reversibly increases cerebrovascular permeability to [14C]sucrose in the right cerebral hemisphere. PA ([14C]sucrose permeability x capillary surface area) rises from a control mean of 11 x 10(-6) S-1 to above 200 x 10(-6) S-1. The rise correlates with an increased staining of the brain by intravascular Evans blue, and is followed by a transient, 1-1.5% increase in brain water content. At least 20 s of infusion is required for 1.6 M arabinose solution to effectively open the blood-brain barrier. The increase in cerebrovascular permeability is temporary, however, because PA remains slightly elevated 1-2 h after infusion and is normal 6 h after infusion. It is suggested that osmotic barrier opening is mediated by cerebrovascular dilatation as well as by shrinkage of the vascular endothelium. By quantitatively defining thresholds of infusate concentration and infusion time for osmotic barrier opening, and by characterizing the time course of increased PA, the experiments establish criteria for applying the osmotic method to experimental pharmacology of the central nervous system.