Two methods were used to induce a model of experimental hydrocephalus and hydrosyringomyelia in the cat: a) injection of kaolin into the cisterna magna and b) closure of the lateral apertures of the fourth ventricle with cotton swabs. The pathological changes in the brain ventricles and the central canal of the spinal cord were monitored and documented at regular intervals by computed tomography (CT). The CT method is particularly advantageous for studies of this kind because the animals can be examined frequently without risking disturbances in cerebro-spinal fluid dynamics or tissue damage that would result from introduction of contrast media into the ventricular system. Our results and others reported earlier suggest that the dilated central canal acts as a kind of spontaneous CSF deviation route from the ventricles to the subarachnoid space. In spite of the tendency of the animals to recover in a clinical sense, the internal CSF space continued to expand; in cats the disease is progressive, a fact that is readily evident in follow-up CTs. This characteristic indicates that the spontaneous "shunt system" from the fourth ventricle through the dilated central canal to the spinal CSF spaces does not function well enough. Possible explanations for hydrocephalus compensation and the development of hydrosyringomyelia in experimental animals are discussed.