Glycolysis has been evaluated in situ in chick embryo cell cultures transformed with Rous sarcoma virus. No characteristic difference in lactic acid production was observed between normal and transformed cultures in a state of optimal growth. Under certain conditions chosen to limit cell multiplication, glycolysis declined commensurate with growth rate in both control and transformed cultures. However, cultures infected with Rous sarcoma virus were less inhibited in this respect than their uninfected counterparts. Thus, transformed cells showed a differential increase (i.e., less inhibition) in rates of lactate production and protein accretion under conditions of contact inhibition, serum depletion, thymidine block, and culture in liquid suspension medium. These differences were generally evident within two days of viral infection. It was concluded that transformation of chick embryo cells by Rous sarcoma virus enhanced their capacity for growth and that augmented glycolytic rates appeared only as this differential growth potential was expressed.