Labelled red blood cells were instilled intraventricularly in groups of dogs and monkeys for evaluation of transfer to the vascular compartment. Blood levels were minimal (5% of the original material) with low-pressure infusions. A large movement (above 25%) of the type originally described by Simmonds occurred with high-pressure intraventricular infusions, cisternal injections, or after the induction of intracranial hypertension. Large amounts of the label were released after cell lysis and accumulated in various tissues. Five to ten per cent of the instilled material was absorbed by the nasal lymphatic route in the dog and a significant local loss of red cells occured after posterior cisternal punctures. Simulated whole blood was useful in demonstrating arachnoid retention. Ventriculospinal washouts aided in the elimination of tagged cells only in the first 24 hours. Thereafter, the yield was small and the fluid requirement was unusually high.