&NA;We have developed a noninvasive method to determine the flow rates necessary to produce hyperosmotic blood‐brain barrier disruption in individual animals. The method uses the intracarotid infusion of 0.9% NaCl at increasing flow rates, while concurrently measuring the amplitude of the somatosensory evoked potential (SSEP). For these studies, dogs were prepared with craniotomies to visualize the duration of cortical blanching. Saline (0.9% NaCl) was infused into the internal carotid artery at flow rates of 0.9 to 2.1 ml/s for periods from 2 to 40 seconds while the SSEP was concurrently measured in six 30‐second epochs before, during, and after the infusion. There was a linear relationship between the duration of cortical blanching (from 2 to 30 seconds) and amplitude suppression of the major negative deflection of the SSEP. This relationship allowed us to predict the intracarotid infusion rate necessary to achieve cortical blanching for periods of 20 seconds or more. Subsequent infusion of 1.4 or 1.6 osmolar mannitol at the predicted flow rate resulted in more pronounced suppression of the SSEP than did 0.9% NaC1 and produced disruption of the blood‐brain barrier as documented by computed tomographic scans with contrast enhancement. Intracarotid infusions of 0.9% NaC1 that produced partial blanching (partial replacement of blood in cortical vessels) were associated with paradoxical enhancement of SSEP amplitude, thus providing an additional measure of the adequacy of the infusion rate. This technique provides a reliable method to determine intracarotid infusion rates for hyperosmotic disruption of the blood‐brain barrier in individual animals, before the intracrotid infusion of hyperosmolar solutions. (Neurosurgery25:405‐411, 1989)