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Stability of mammalian cell volume depends primarily on the sodium pump. When active cation transport of rabbit renal proximal tubules is blocked by ouabain, cells swell, but their size is limited by residual volume control mechanisms. This "ouabain-resistant" volume control is not an active process, as it operates in the presence of cyanide and dinitrophenol and in the absence of exogenous energy. Nevertheless, it remains incompletely explained by known transmembrane oncotic and hydrostatic forces. We tested the hypothesis that the cytoskeleton contributes to isotonic cell volume control. Isolated, collapsed rabbit proximal convoluted tubules (PCT) were crimped at both ends with micropipettes and had their volume assessed optically. PCT in ouabain (1 mM) swelled to 1.40 above control with protein, 1.62 without protein, and 1.89 with the cytoskeleton inhibitors vincristine (5 microM) and cytochalasin B (50 microM) and without protein. Tubulozole-C and cytochalasin D gave similar results. A hydrostatic pressure of 50 cmH2O increased tubule volume to 1.93 before the tubule basement membrane (TBM) prevented further volume increase. We conclude that volume of renal tubule cells in ouabain is limited partly by external protein, but primarily by the cytoskeleton. The TBM prevents massive swelling and tubule disaggregation.