The effect of K+ depletion of Hep 2 cells on ion fluxes, internal pH, cell volume, and membrane potential was studied. The cells were depleted of K+ by incubation in K+-free buffer with or without a preceding exposure to hypotonic medium. Efflux of K+ in cells not exposed to hypotonic medium was inhibited by furosemide or by incubation in Na+-free medium, indicating that in this case at least part of the K+ efflux occurs by Na+/K+/Cl- cotransport. After exposure to hypotonic medium, K+ efflux was not inhibited by furosemide, whereas it was partly inhibited by 4,4'-diisothiocyano-2,2'-stilbene-disulfonic acid (DIDS). Exposure to hypotonic medium induced acidification of the cytosol, apparently because of efflux of protons from intracellular acidic vesicles. When isotonicity was restored, a rebound alkalinization of the cytosol was induced, because of activation of the Na+/H+ antiporter. While hypotonic shock and a subsequent incubation in K+-free buffer rapidly depolarized the cells, depolarization occurred much more slowly when the K+ depletion was carried out by incubation in K+-free buffer alone. The cell volume was reduced in both cases. K+ depletion by either method strongly reduced the ability of the cells to accumulate 36Cl- by anion antiport, and K+-depleted cells were unable to increase the rate of 36Cl- uptake in response to alkalinization of the cytosol.