The morbidity and mortality of subarachnoid hemorrhage (SAH) is directly related to blood in the subarachnoid space. Blood was injected intracisternally in 10 monkeys, and the absorption resistance of cerebrospinal fluid (CSF) was studied. The monkeys given injections of nonheparinized blood maintained high absorptive resistances, whereas the resistances of the group that received heparinized blood returned to the original levels. If heparinization of the CSF can be achieved rapidly after SAH and the deposition of fibrin clots along the pathways of CSF drainage can be prevented, the resorption of blood through the arachnoid granulations might be facilitated.