Renal tubule cell volume is thought to be kept constant by a cation pump. Ouabain, by inhibiting Na+-K+-ATPase, blocks cation transport with resultant cell swelling, but the degree of swelling is less than expected were active cation transport completely inhibited. Although the relatively rigid tubule basement membrane may limit swelling of ouabain-treated tubules, some investigators have alternatively postulated that an energy-dependent ouabain-insensitive cation pump regulates cell size. This notion derives from studies of renal cortical slices in which metabolic inhibitors such as 2,4-dinitrophenol (DNP) cause more swelling than ouabain. Cellular metabolism of isolated rabbit proximal straight tubules was blocked by adding metabolic inhibitors to or removing acetate and glucose (energy substrate) from the bathing medium and evaluated subsequent changes in cell size by measuring outer diameter of nonperfused tubules. In isotonic medium, cell volume increased 36% with addition of 10-4 M ouabain, 40% with 10-2 M DNP, 46% with 10-3 M cyanide, 39% with ouabain + DNP + cyanide, and 37% with removal of both substrate. Apparently renal tubule cell volume is not regulated by a unique ouabain-insensitive cation pump.