The normal cerebrospinal fluid flow as monitored by serial CT cisternography with metrizamide (Amipaque) is described in 25 individuals. Preliminary gross autoradiography using 131I-labelled metrizamide concur with these CT cisternographic findings indicating that intrathecally introduced metrizamide penetrates the brain substance. Adverse reactions are most prominent during the periods of maximum brain penetration and include headache, nausea, perceptual aberrations, and EEG alterations. The value and diagnostic applications of cerebral and cerebellar penetration of metrizamide are also discussed.