The concentration of potassium in the extracellular fluid has been found to stimulate the rate of CO2 fixation by astroglial cells grown in primary culture. Raising the concentration of extracellular potassium increased both the initial rate of formation of the 14C-labeled products of 14CO2 fixation and the final steady-state level of these products within the cells. In contrast, neither veratridine nor L-glutamate affected the rate of CO2 fixation in astroglial cells. The very low rate of CO2 fixation found in primarily neuronal cultures was unaffected by increased extracellular potassium as was CO2 fixation in fibroblasts. When cultured alone, astroglial cells release a large fraction of the 14C-labeled products of CO2 fixation into the surrounding medium. Mixed cultures of astroglia and neurons also fix CO2 but, in contrast to astroglia cultured alone, release only a small fraction of the 14C-labeled products into the culture medium.