Quiescent C3H-10T1/2 mouse fibroblasts that have not undergone any type of stress have a relatively low rate of 2-aminoisobutyrate (Aib) uptake by means of system A, which is primarily energized by the transmembrane Na+ chemical gradient potential. System A activity in these cells is not sensitive to ouabain or proton ionophores. In contrast, methylcholanthrene-transformed and confluent C3H-10T1/2 cells treated with 0.4 mM ouabain for 16-20 hr utilize the membrane potential generated by the Na+, K+-ATPase pump to drive Aib transport by means of system A as shown by the sensitivity of transport activity to ouabain and proton ionophores. Since glucose is present during the assay, the proton ionophores do not affect the availability of ATP, as indicated by the undiminished uptake of 86Rb+ by the Na+, K+-ATPase pump. As cells progress through the G1 phase of the cell cycle, they show an increased system A activity prior to entry into the S phase, which is also dependent on the electrogenicity of the Na+, K+-ATPase pump. There appears to be in all these cases a qualitative shift in the bioenergetic mechanism for the uptake of Aib as well as a marked quantitative increase in Aib uptake. The high activity after ouabain treatment was sustained in the transformed cells after removal of the ouabain, whereas in the confluent 10T1/2 cells the rate of uptake decayed rapidly, suggesting a difference in the mode of regulation. We conclude that transformed cells and normal cells in late G1 or under stress make use of the membrane potential generated by the Na+, K+-ATPase pump to drive amino acid uptake by means of system A.