The acid-base balance (pH, pCO2 and HCO-3) of 23 normal subjects was determined both in arterialized capillary blood and in the CSF. 2. Statistically significant correlations (determined by means of Spearman's rank correlation) were found between: pCO2 in arterialized blood and CSF pH (Rs=--0.372, p is less than 0.05), pCO2 in the CSF and CSF pH (Rs=--0.421, p is less than 0.05), HCO-3 in the CSF and in arterialized blood (Rs=0.623, p is less than 0.05), blood pH and CSF pH (Rs=0.485, p is less than 0.025), pCO2 in the CSF and HCO-3 in the CSF (Rs=0.559, p is less than 0.005). 3. The regulatory mechanisms of the CSF acid-base balance in normal subjects and also in patients with extra-neural or CNS disturbances are discussed.