Potassium ion pool was studied in glycolyzing Enterococcus hirae, grown at high or low alkaline pH (pH 9.5 and 8.0, respectively). Energy-dependent increase of K+ pool was lower for the wild-type cells, grown at pH 9.5, than that for the cells grown at pH 8.0. It was inhibited by N,N'-dicyclohexylcarbodiimide (DCCD). The stoichiometry of DCCD-inhibited K+ influx to DCCD-inhibited H+ efflux for the wild-type cells, grown at pH 9.5 or 8.0, was fixed for different K+ external activity. DCCD-inhibited ATPase activity of membrane vesicles was significantly stimulated by K+ for the wild-type cells grown at pH 9.5, and required K+ for the wild-type cells grown at pH 8.0, while the levels of alpha and beta subunits of the F-1 and b subunit of the F-0 were lower for the cells grown at pH 9.5 than that for the cells grown at pH 8.0. Such an ATPase activity was residual in membrane vesicles from the atpD mutant with a nonfunctional F0F1. ATPase activity of membrane vesicles from the mutant with defect in Na+-ATPase was higher for the cells grown at pH 9.5 than that for the cells grown at pH 8.0, and was inhibited by DCCD. An energy-dependent increase of K+ pool in this bacterium, grown at a high or low alkaline pH, is assumed to occur through a K+ uptaking system, most probably the Trk. The latter functions in a closed relationship with the H+-translocating ATPase F0F1.