The regional distribution of ethanol in selected areas of the rat brain was studied after single intravenous and intraperitoneal injections. Ethanol concentrations (measured by gas-liquid chromatography) in cortex, striatum, and hippocampus were compared with arterial and venous blood alcohol concentrations. As previously reported, equilibrium between tissue and arterial blood occurred within 3 min and followed simple diffusion kinetics. At shorter time intervals (1 min) after injection, regional ethanol concentrations differed, possibly because of regional blood flow and tissue mass. Equilibrium between tissue and venous blood required 10--15 min and coincided with the disappearance of the arterial-venous difference. These findings suggest that tissue ethanol concentrations cannot be determined from venous blood samples until brain arteriovenous equilibrium has occurred. They also support the argument that alcohol concentrations in tissue perfusates do not necessarily provide a reliable guide to those in the tissue.