The use of CSF resistance determinations as a diagnostic test for acute hydrocephalus appears feasible. Resistance and compliance of the CSF circulation were measured in 25 cats. Kaolin then was infused into the cisterna magna of 20 of these cats and artificial CSF into the cisterna magna of the other five. Repeat resistance and compliance measurements were made on each cat at a predetermined time within a month after the cisternal infusion. The animal was then killed, the brain fixed, and the degree of hydrocephalus estimated from measurements made on lateral ventricles in coronal section. Progressive hydrocephalus developed only in the kaolin-treated cats. Resistance was increased and compliance was normal in the hydrocephalic cats. Resistance decreased as the duration of hydrocephalus increased, implying that progressive hydrocephalus could not be explained by inadequate CSF absorption alone.