The purpose of this study was to measure the changes in potassium concentration in human cisternal cerebrospinal fluid following successful resuscitation after cardiac arrest. We also wished to examine whether or not changes in potassium concentration in the cisternal cerebrospinal fluid could be correlated to the ability to regain normal cerebral function. 41 patients were studied, of whom 20 regained consciousness and 21 did not. In those who did not regain consciousness there was a significant increase in the potassium concentration found in samples obtained between 40 and 50 min, and between 50 and 60 min after cardiac arrest. The potassium concentration decreased to normal values during the following hours. Lumbar spinal fluid did not reflect the changes in cisternal fluid. The results suggest that the potassium concentration of cisternal cerebrospinal fluid, obtained soon after cardiac arrest, might give an indication of the degree of cerebral damage caused by cardiac arrest.