Congenital murine hydrocephalus occuring in the Hy-3 strain has certain similarities to human congenital hydrocephalus. In order to elucidate the pathophysiology and etiology of congenital hydrocephalus, ultrastructural study of the brain and the leptomeninges in the Hy-3 hydrocephalic mice was undertaken. The cerebral cortex was invested with the thick cellular layer which had similar features of primitive arachnoid barrier cells. No appearent subarachnoid space was found. Horseradish peroxidase, introduced into the lateral ventricle, crossed the ependymal cells and penetrated into parenchymal extracellular spaces. However no horseradish peroxidase activity was found over the cerebral hemispheres. Those results suggested that the hydrocephalus in the Hy-3 strain is due to development anomaly of the leptomeninges and to defect of cerebral subarachnoid space.