The ultrastructural changes of the wall of the fourth ventricular foramina following intraventricular autogenous blood infusion present four meaningful findings for the patho-genesis of secondary hydrocephalus. Using the transmission and scanning electron microscopy (TEM and SEM), it has found that minimal to marked separation of the intercellular cleft coincided with the intra-and intercellular vacuolation and swelling of the glial fibers in the subependymal glial sheath by the 7th day of blood infusion. A f1attening of the contours of the ependymal cells and their nuclei was noted under the TEM and a seperation of ependymal cells was pronounced under the SEM during the period between the 28th and 42nd day. Ultrastructural changes of the ependymal cells correlated with the time factor and not with the blood volume infused. The supraependymal cells (SEC) seen on the ventricular surface were indicative of neuron-like structure rather than macrophages.