SUMMARY1. Changes in water and ion contents of renal cortical slices from rat, rabbit and guinea-pig incubated in medium of a half normal osmolarity were followed over 60 min incubation at 25°C.2. Metabolizing slices gained water and lost sodium and chloride but not potassium. These changes reflected in part changes in cellular composition, and were complete within 5 min. A new steady-state was then maintained for a further 55 min.3. Slices initially incubated for 60 min with ouabain, also lost sodium and chloride when transferred to the hyposmotic medium but did not, in the first minutes, lose any further potassium. Ouabain, in concentrations sufficient to produce maximal inhibition of the Na-K ATPase, did not cause additional cellular swelling in the hyposmotic medium. 4. The results offer no support for the claim that cellular volume in metabolizing renal cortical cells exposed to hyposmotic media is regulated by loss of cellular potassium. They suggest that under such conditions the regulation of volume involves the same ouabsin-insensitive mechanism as is seen in cells incubated in isosmotic medium.