The rate of CSF production in cats was studied in relation to changes in intracranial pressure (ICP), systemic arterial pressure (SAP), and the resultant alterations in cerebral perfusion pressure (CPP). When ICP is raised to the point where CPP falls below 55 mm Hg, CSF production will diminish. Elevation of ICP does not appear to influence CSF production rate if CPP is maintained above 70 mm Hg.