As normal chick embryo (CE) cells entered quiescence the intracellular concentrations of both Na+ and K+ declined. Comparable decreases in intracellular concentrations of Na+ and K+ were not observed in CE cells transformed by either the Schmidt-Ruppin (SR) or the Bryan (B) strain of Rous sarcoma virus (RSV). Intracellular concentrations of Na+ were higher in SR-RSV-transformed CE cells than in B-RSV-transformed cells and uninfected CE cells at all times after plating. In contrast, intracellular concentrations of K+ were higher in B-RSV-transformed CE cells than in SR-RSV-transformed cells. Uninfected CE cells incubated in medium containing an elevated concentration of K+ (an increase from 5 to 30 mM) exhibited several, but not all, of the transformation parameters expressed by B-RSV-transformed CE cells.