The presence of pH-sensitive transport mechanisms in the basolateral membrane of proximal tubular cells suggests that cell volume and its regulation may be sensitive to changes in cell pH. We have measured the response of cell pH and cell volume to changes in the acid-base composition of solutions bathing isolated, lumen-collapsed, proximal S2 tubular segments taken from the rabbit kidney. Cell pH was determined by measurement of the fluorescence emission of 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. Cell volume was calculated from measurements of tubular diameter. An increase in CO2 from 5 to 15% reduced cell pH 0.30 units and raised cell bicarbonate concentration ([HCO3]) 10 mM. Cell volume rose to 108.6% of control in 4 min. A decrease in bath [HCO3] from 25 to 5 mM reduced cell pH 0.41 units and cell [HCO3] by 15 mM. Cell volume gradually increased to 105.7% at 8 min. The rate of the regulatory volume decrease after cell swelling on exposure to a 160 mosM solution was determined in the presence of 5 and 15% CO2. The latter reduced the maximum fractional rate of recovery of volume from 0.18 to 0.11 min-1 but did not affect the extent of regulation. We conclude that acidosis causes cell swelling and reduces the rate of volume regulation in response to hypotonic media.