Fifteen young guineapigs were made hydrocephalic by intracisternal injection of kaolin and 16 guineapigs were used as control. Volume, number and surface areas of both the outer and inner membranes of the mitochondria of the choroidal ependyma of these animals were studied, using a coherent multipurpose test system. Volume, number and surface area of the outer membrane of the mitochondria did not show any significant difference between the two groups. However, surface area of the inner membrane of the mitochondria including cristae showed a significant decrease in the hydrocephalic animals. This reduction in the surface area could probably be attributed to the reduced activity of the choroidal ependymal cells in obstructive hydrocephalus.