Multiple simultaneous isotoplc tracer studies were carried out in patients whose 1st 3 ventricular compartments were isolated from the subarachnoid space. Water and electrolytes were found to enter the cerebrosplnal fluid (CSF) rapldly both in the ventricles and throughout the subarachnoid space. The formation of the fluid is thus not an exclusive prerogative of the choroid plexus. Water and electrolytes enter the blood directly from both the ventricles and the subarachnoid space, doing so at roughly similar rates. CSF does not suddenly come into being fully formed at any point with all its constituents in their proper ratios, but each constituent is exchanging with blood at its own characteristic rate. In contrast to water and electrolytes, protein is absorbed largely from the subarachnoid space, presumably from the arachnoidal villi. By virtue of this specific role played by the arachnoid villi, they serve a function in the CSF system analogous to the lymphatics of the general circulation. The site and rate of protein reabsorption is an important determinant of the direction and rate of CSF flow. The net amt. of CSF elaborated per day in the ventricles, over and above the large and equivalcnt vol. exchanged, is small, and is of the order of 10 to 20 cc.or less per day in man.