Oral glycerol was administered to eight patients with meningeal carcinomatosis or acute leukemia in whom ventricular catheters and Ommaya reservoirs had been implanted for the purpose of intrathecal chemotherapy or chemoprophylaxis. Intraventricular pressure was monitored continuously via the Ommaya reservoirs before and after single doses of 0.5, 1.0 or l.5 gm per kilogram of body weight. The interrelationship between initial pressure, change in pressure, serum osmolarity, and duration of action was investigated, and the ratio of CSF-to-plasma osmolarity was determined 4 to 5 hours after glycerol administration. The effects of chronic 6-hourly and 4-hourly 1 gm per kilogram glycerol doses were studied in a patient with meningeal carcinomatosis and increased intracranial pressure. Our data suggest that as a cerebral dehydrating agent oral glycerol is most effective in patients with markedly increased intracranial pressure. A single 1 gm per kilogram dose is adequate to lower raised intraventricular pressure acutely, but its effect is short-lived. Continuous oral administration must be carefully monitored to avoid the establishment or a reverse osmotic gradient, secondarily increased intracranial pressure, and clinical deterioration.