The uptake of K(+) and Ca(2+) in Dunaliella salina is mediated by two distinct carriers: a K(+) carrier with a high selectivity against Na(+), Li(+), and choline(+) but not towards Rb(+), K(+), Cs(+), or NH(4) (+), and a Ca(2+) carrier with a high selectivity against Mg(2+). The latter is specifically blocked by La(3+) and by Cd(2+). Apparent K(m) values for K(+) and Ca(2+) uptake are 2.5 and 0.8 millimolar, respectively, and their maximal calculated fluxes are 22 and 0.8 nanomoles per square meter per second, respectively. Effects of permeable ions and ionophores on K(+) and Ca(2+) uptake suggest that the driving force for their uptake is the transmembrane electrical potential. Inhibitors of ATP production, typical inhibitors of plasma membrane H(+)-ATPases and protonionophores inhibit K(+) and Ca(2+) uptake and accelerate K(+) efflux. The results suggest that an H(+)-ATPase in the cell membrane provides the driving force for K(+) and Ca(2+) uptake. Efflux measurements from (86)Rb(+) and (45)Ca(2+) loaded cells suggest that part of the intracellular K(+) and most of the intracellular Ca(2+) is nonexchangeable with the extracellular pool. Correlations between phosphate and K(+) contents and the effect of phosphate on K(+) efflux suggest intracellular associations between K(+) and polyphosphates. On the basis of these results, it is suggested that: (a) K(+) and Ca(2+) uptake in D. salina is driven by the transmembrane electrical potential which is generated by the action of an H(+)-ATPase of the plasma membrane. (b) Part of the intracellular K(+) is associated with polyphosphate bodies, while most of the intracellular Ca(2+) is accumulated in intracellular organelles in the algal cells.