S udden alteration in medium osmolality causes an osmometric change in proximal tubule cell size followed by restoration of cell volume toward normal in hypotonic but not in hypertonic medium. We determined the capability of isolated nonperfused proximal tubules to prevent a change in cell volume in anisotonic media. The external osmolality was gradually changed over a range from 110 to 480 mosM. At 1.5 mosM/min, cell volume remained constant between 167 +/- 9 and 361 +/- 7 mosM, a phenomenon termed isovolumetric regulation (IVR). Cells lost intracellular solutes in hypotonic and gained intracellular solutes in hypertonic media. Raffinose or choline chloride substitution showed that osmolality, rather than NaCl, signalled cell volume maintenance in hyperosmotic media. Cooling (7-10 degrees C) blocked IVR. IVR was maintained when osmolality was lowered at a rate of 27, but not at 42 mosM/min. IVR was not observed when the rate of osmolality increase exceeded 3 mosM/min. We conclude that proximal tubule cells sensitively regulate intracellular volume in an osmolality range of pathophysiologic interest by mechanisms dependent on the rate of net water movement across basolateral membranes and the absolute intracellular content of critical solutes.