Glycolyzing cells of Streptococcus faecalis accumulate K(+) with concurrent extrusion of equivalent amounts of H(+) and Na(+). An attempt was made to clarify the retionship between the movements of Na(+) and K(+). Sodium was displaced from cells glycolyzing in the presence of ammonia, diethylamine, tris(hydroxymethyl)aminomethane, and other nitrogenous cations; by contrast, K(+) was completely retained. Accumulation of K(+) by heterologous exchange for Na(+) was not inhibited by antibiotics which facilitate diffusion of K(+) across the membrane, but was blocked by proton conductors. The results indicate that extrusion of Na(+) and H(+) from the cells is a primary, energy-linked process which generates an electrical potential (interior negative); K(+) accumulation occurs in response to this potential. Two mutants deficient in K(+) accumulation and retention were examined in terms of this model. One mutant is apparently defective in exchange of K(+) for H(+). In the other mutant, exchange of K(+) for Na(+) is impaired.