PRECAPILLARY arteriovenous anastomoses are a normal part of the microcirculatory bed of skin, muscle, lung, heart, intestine, liver, spleen, kidney, ear, and eye.1,2 These have been labeled "preferential channels"3 or "throughfare channels" by Chambers and Zweifach.4 Whether similar anastomoses occur in the brain parenchyma has been a subject of debate. Rowbotham and Little5-9 observed precapillary arteriovenous anastomoses in the pial circulatory system but not in the brain parenchyma of man, while Pfeifer10,11 and Solnitsky12 saw them in both. Benda and Brownell13 and Swank and Hain14 injected particles up to 60μ in diameter into the carotid arteries of dogs and observed that they passed to the cerebral veins. Because the average capillary diameter has been found to be 5.8μ by Ueda,15 the passage of large emboli from artery to vein means that there is a shunt bypassing the capillaries or that