ABSTRACT It is quite usual to see clear fluid collecting continuously on the surface of the spinal cord of ‘functionally decapitate’ cat preparations which have no functioning choroid plexus. The activity of potassium and calcium ions in interstitial fluid of these decapitate spinal cords is equal to that in spinal cords of cats with intact choroid plexus function, similar to normal cerebrospinal fluid, and different from blood plasma. It follows that fluid is being formed within the spinal cord by a process different from ultrafiltration. Endothelial cells seem likely to form this interstitial fluid by actively transporting Na+, with water following by osmotic force. Potassium and other ion concentrations may be adjusted by independent transport processes. A role could be attributed to glial cells in these transports if exchange of material could be demonstrated between glia and endothelium.