The rate of glucose uptake and the formation of lactic acid and CO₂ were studied in normal and Rous sarcoma virus-transformed chick-embryo fibroblasts cultured at low and high cell densities. In normal cells the ratio of lactic acid produced to CO₂ evolved increased with increasing cell density. These changes were not due to variations in the pool sizes or breakdown of glycogen but indicated high cell-density cultures preferred use of the glycolytic pathway over aerobic pathways. The phosphogluconate pathway also became more prominent as cells became relatively dense. However, population density per se and not the growth rate appeared responsible for the changes in paHern of carbon flow. Transformed cultures were similarly affected by population density. Moreover, under comparable conditions of growth rate and population density, the rate of lactic acid production and the ratio of lactate/CO₂ were higher in transformed than in normal cultures. These observations indicated that viral transformation of chick-embryo fibroblasts in culture causes alterations in glucose metabolism in addition to those ascribable to increasing cell density. Cells infected with a nontransforming Rous-associated virus had metabolic paHerns similar to normal cultures.