The number of cells in the brain and their size can be calculated by determining the total brain DNA, RNA and protein. Myelin can be determined biochemically. This analysis was applied to brains of a group of adult cats with craniectomies that were made severely hydrocephalilc by the introduction of kaolin or silicone into the basal cisterns. In severely hydrocephalic brains sectioned rostral to the area of kaolin inflammation there was no loss of dry weight. Wet weight increased slightly reflecting periventricular edema. There was a slight increase in total DNA, RNA and protein, presumably reflecting the increase in small inflammatory cells. There was a significant reduction in galactolipids reflecting myelin loss.