The effect of lactate on glucose metabolism in neurons and astrocytes from primary culture has been studied. The rates of glucose metabolism through the pentose-phosphate shunt, the pyruvate dehydrogenase-catalyzed reaction, the tricarboxylic acid cycle, the total lipogenesis and the synthesis of glycerol-borne lipids in astrocytes were 2-3 fold higher than in neurons. However, the rate of glucose incorporation into sterols and esterified fatty acids was similar in both types of cells. Total glucose utilization was inhibited by lactate to the same extend in both neurons and astrocytes. Lactate strongly inhibited glucose oxidation through the pyruvate dehydrogenase-catalyzed reaction and the tricarboxylic acid cycle, in both neurons (60 and 44%, respectively) and astrocytes (64 and 62%, respectively). Glucose incorporation into sterols and fatty acids was also inhibited by lactate in both neurons and astrocytes (57 and 76%, respectively) while the oxidation of glucose in the pentose-phosphate shunt and the synthesis of glycerol-borne lipids was not significantly affected. These results suggest that in the presence of lactate both neurons and astrocytes can utilize lactate as the major metabolic substrate, sparing glucose for the synthesis of NADPH(H+), ribose-5-phosphate and/or glycerol-borne lipids. An interaction between glucose and lactate metabolism at the level of the pyruvate dehydrogenase-catalyzed reaction is suggested.