Thorotrast has recently been advocated1as a suitable substance for intracranial injection for diagnostic purposes. Freeman and his associates expressed the belief that the toxic effect is less than that of air and that the substance mixes readily with spinal fluid but is somewhat heavier and therefore visualizes more clearly dependent recesses of the cerebrospinal fluid spaces. We have been studying its reactions in animals for several months and find it to be a dangerous drug. In moderate doses injected intracranially it produces hydrocephalus, and it concentrates in dependent parts. It is dangerously radioactive, and its elimination from the cerebrospinal fluid spaces is insignificant. Thorotrast 1073D is a stabilized colloidal suspension of thorium dioxide prepared for intravenous or intracranial use. The original product was modified by Wustmann,2with the cooperation of the Heyden Chemical Corporation. At first it was not well buffered and produced severe toxic effects when