In an effort to determine which blood elements play a critical role in the development of cerebral vasospasm, adult cats were subjected to prepontine injection of either autogenous whole blood or erythrocyte-free blood containing latex beads as a substitute morphologic marker. Seven or 10 days later the cats were anesthetized and perfused with fixative, and the basilar arteries were prepared for light and electron microscopy. Successful clot deposition was confirmed by the presence of numerous erythrocytes or latex beads within the adventitia of vessels. In agreement with previous studies, instillation of whole blood produced luminal narrowing associated with profound ultrastructural changes in all layers of the vascular wall. No significant alterations, however, occurred in arteries bathed in erythrocyte-free blood. These findings suggest first, that erythrocytes are essential for the development of the vasculopathy associated with chronic cerebral vasospasm, and second, that the role, if any, of other blood elements is not autonomous.