One group of cats had an acrylic screw implanted into the adqueduct of Sylvius, while the other group of animals received a solution of kaolin into the cisterna magna. Three weeks later the dye phenolsulphonphthalein was instilled into the lateral ventricle to ascertain communication between CSF compartments, and thereafter the brain was perfused with formalin. As shown by planimetry of brain ventricles both groups of experimental animals developed hydrocephalus, i.e., coronal surface of brain ventricles was about 10 times larger in kaolin and about 3 times in aqueductal screw experiments than in the controls, respectively. In aqueductal screw experiments communication of CSF between lateral ventricle and subarachnoid spaces was not blocked but only restricted, i.e., an aqueductal stenosis was produced. In kaolin experiments communication of CSF between lateral ventricles and spinal subarachnoid space was blocked by thick meningeal adhesions in the upper cervical region, while the central spinal canal was dilated (hydromyelia) with enhanced CSF communication between it and the lumbar subarachnoid space. We assume that during systolic expansion of brain the CSF is displaced from the cranial cavity toward the spinal subarachnoid space which accommodates an additional volume of CSF primarily due to compliance of the spinal dural sac, while during diastole CSF recoils in the opposite direction. Thus, in case of aqueductal stenosis the undisplaced volume of CSF from the ventricles can be accommodated due to diminution of cerebral blood volume and brain parenchyma so that hydrocephalus develops over time. Since the cervical subarachnoid space is blocked in kaolin experiments the systolic brain expansion forces CSF from basal cisterns via the fourth ventricle into the aqueduct and central canal with consequent development of hydrocephalus and hydromyelia.