Hydrocephalus was induced in 13 dogs by injecting kaolin into the cisterna magna and was evaluated by computerized tomography (CT) scans. Modification of periventricular hypodensity was observed by metrizamide-enhanced CT ventriculography. Periventricular hypodensity was seen as early as 12 hours after kaolin injection. On CT ventriculography, metrizamide stayed longer in the ventricles of hydrocephalic dogs than in those of normal dogs, and migrated into the areas of periventricular hypodensity; the changes became significant within 12 to 24 hours. Four of the dogs were killed immediately after CT ventriculography, and the iodine concentration was measured. Iodine concentration was highest in the periventricular white matter, followed by the basal ganglia, and it was low in the cerebral and cerebellar cortex. When the change in Hounsfield units found by CT ventriculography at the regions of interest was compared to the actual iodine concentrations, the figures were quite compatible. Similarly, the specific gravity was measured in tissue from various parts of the brain of two hydrocephalic dogs, and compared against the value of that from five normal dogs. The specific gravity values were particularly low in the periventricular white matter of the hydrocephalic brains, suggesting a higher water content in that region. Since the increased migration of metrizamide occurred at the same region, it is suggested that development of periventricular hypodensity is due to increased transit of cerebrospinal fluid from the ventricles to the white matter.