The rate at which chick embryo fibroblasts in primary or secondary culture transport glucose or 3-O-methyl glucose is strongly influenced by the presence of bicarbonate ion in the culture medium. Cells growing or maintained on glucose at physiologic concentration (5.5 mM) have an 8 to 10 fold higher rate of glucose uptake than their counterparts cultivated without bicarbonate. These cells also produce more lactate as a consequence of their more rapid intake of glucose. The hydrogen acceptors, methylene blue and dehydroascorbate added to the culture medium reduce the cell capacity to transport glucose and 3-O-methyl glucose to levels obtaining in the bicarbonate-free medium. There is a concomitant reduction in glucose utilized by cells during 24 hours and further reduction in lactate formed per molecule of glucose metabolized.