Oxygen consumption was measured in mouse L-929 cells whose volumes and water contents were reduced by adding sorbitol to the medium. The volume of water lost due to a given sorbitol supplement exceeded the loss in apparent cell volume. An explanation is given for this discrepancy. The rate of oxygen uptake in the absence of exogenous respiratory substrate was essentially the same in cells whose total volume was reduced by 45%, amounting to a loss of about 70% of the total cell water, compared to controls at 'physiological' volume and water content. Cells under these same conditions responded to added substrates (pyruvate, glucose, and glutamine) and inhibitors (iodoacetate and 2-deoxyglucose) in nearly the same way as control cells. These observations are in accord with and add to previous work showing that very large fluctuations in cell volume and water content have only modest effects on the rates and directions of a variety of metabolic processes. The results are interpreted in terms of current views on the composition and organization of the aqueous compartments of eucaryotic cells.