1. The effect of ouabain on cellular volume recovery in rabbit, guinea-pig and rat renal cortical slices was studied. A concentration of ouabain that is maximally effective in inhibiting slice potassium accumulation was determined for each species. Slices from each species were either freshly prepared and then incubated, or leached and then incubated, or preincubated in oxygenated ordinary medium (equilibrated), leached and then reincubated in media with and without this concentration of ouabain. All incubations were at 25 degrees C.2. Potassium loss produced by ouabain was greater in rabbit and guinea-pig slices than in rat slices.3. With slices that were freshly prepared and then incubated, and with slices that were leached and then incubated, cellular volume recovery was inhibited by ouabain in rabbit and guinea-pig slices, but not in rat slices.4. After equilibration, swelling during leaching was less, especially in rabbit and guinea-pig slices. However, on subsequent reincubation, significant differences in tissue water and cation contents that were consistent with inhibition of cellular volume recovery by ouabain, were seen in slices from these two species, but not in rat slices.5. Slices from all three species, when incubated with concentrations of ouabain that were maximally effective in inhibiting potassium accumulation, appeared to approach a steady-state tissue potassium content that was greatest in rat slices and least in rabbit slices. Rat slices, previously depleted of potassium, reaccumulated potassium in the presence of 10 mm-ouabain to reach this steady-state potassium content.6. Despite superficial appearance to the contrary (especially in the case of rat slices) these results are consistent with a major role for the conventional pump in controlling cortical cell volume. They do not provide evidence for the postulate that renal cortical cells possess a separate, ouabain-insensitive mechanism regulating cell volume.