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1. The cells in slices cut from the renal outer medulla of normally hydrated adult rats were loaded with Na and depleted of K by incubation for up to 100 min in cold iso-osmolal K-free Ringer containing 180 mM-Na. There was a continuous net cellular water loss during this time; an inverse linear relationship existed between water content and intracellular Na concentration. 2. The original intracellular Na and K concentration were restored following 60 min re-incubation in warm Ringer (37 degrees C) containing 5-9 mM-K. Restoration of cellular water content was incomplete after re-incubation for up to 120 min. 3. During incubation in cold K-free Ringer the presence of 1 mM ouabain did not affect cellular Na uptake or K and water loss. Ethacrynic acid, 1 mM, completely blocked cellular Na uptake and water loss, without affecting the intracellular K concentration at 100 min. When ouabain and ethacrynic acid were present together water loss was also prevented but intracellular Na concentration rose slightly by 100 min. 4. During re-incubation in warm K-containing Ringer 1 mM ouabain inhibited Na extrusion completely for up to 60 min while only partially preventing K uptake and further depressing the level of cellular hydration. Ouabain in the presence of 1 mM ethacrynic acid had similar effects on intracellular Na and K concentrations, but raised the level of intracellular water above that of cells in control slices. 5. Ethacrynic acid alone, 1 mM, did not interfere with Na extrusion or K uptake, but also raised intracellular water above control values. 6. The results obtained are discussed in relation to (a) the nature of the preparation used, (b) the possible membrane transport processes occurring and their known or suggested sensitivity to ouabain and ethacrynic acid, (c) the mechanisms which may be responsible for cell volume maintenance in the medulla.