A technic is described for observing directly with the high powers of the microscope the behavior of the arterioles, capillaries, and venules of the rabbit's ear in the anaphylactic reaction. Such observations show that, in a rabbit sensitized to horse serum, intravenous injection of this antigen, or its introduction by way of the moat, may be followed by: (a) arteriolar contraction, with obliteration of the lumina of the arterioles and stoppage of the circulation, (b) increased adherence of leukocytes to the blood vascular endothelium, and leukocytic emigration through the walls of the capillaries and venules in large numbers, and (c) sticking of leukocytes to each other to form clumps, or emboli, which block the circulation in many capillaries and venules. Under some conditions all of these reactions may follow a single injection of horse serum, either intravenously or directly into the moat. Under others, arteriolar contraction may occur in the absence of extensive leukocyte-sticking, emigration, or the formation of leukocytic clumps. The repeated introduction of serum into the moat causes emigration of leukocytes in increasing numbers from the capillaries and venules, the formation of leukocytic clumps, extravasation of erythrocytes, and finally, in localized regions, endothelial destruction. Such destruction is followed by growth of new blood capillaries into the injured regions, even in the continued presence of serum. No contraction of the capillaries or venules occurred during any of the reactions. Although the blood in the vessels could be very clearly seen with the microscope, no specific precipitate was observed at any time.