INITIAL STUDIES in unanesthetized rhesus suggested a rapid exchange of potassium between brain and CSF.1,2 In order to explore this relationship quantitatively, the exchange of potassium between blood, CSF, and brain has been measured in anesthetized cats, utilizing a technique of continuous perfusion of the animals' ventriculocisternal system with synthetic CSF.3-7 By adding potassium chloride K 42 (42K) to either the perfusate or blood it is possible to calculate the flux of42K into or out of the CSF.3,4 Since brain potassium exchanges very slowly with plasma K,5 it is possible to determine the flux into CSF of potassium from brain and blood separately. These measurements provide information regarding the mechanisms present in mammalian species including man which maintain CSF potassium within the range of 2.6 to 3.2 mM despite variations in serum potassium.6,8,9 In the present experiments the exchange of CSF