Using the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)- carboxyfluorescein (BCECF), we examined the effect of hyperosmolar solutions, which presumably caused cell shrinkage, on intracellular pH (pH(i)) regulation in mesangial cells (single cells or populations) cultured from the rat kidney. The calibration of BCECF is identical in shrunken and unshrunken mesangial cells if the extracellular K+ concentration ([K+]) is adjusted to match the predicted intracellular [K+]. For pH(i) values between ~6.7 and ~7.4, the intrinsic buffering power in shrunken cells (600 mosmol/kgH2O) is threefold larger than in unshrunken cells (~300 mosmo]/kgH2O). In the nominal absence of CO2/HCO3/-, exposing cell populations to a HEPES- buffered solution supplemented with ~300 mM mannitol (600 mosmol/kgH2O) causes steady-state pH(i) to increase by ~0.4. The pH(i) increase is due to activation of Na+/H+ exchange because, in single cells, it is blocked in the absence of external Na+ or in the presence of 50 mM ethylisopropylamiloride (EIPA). Preincubating cells in a Cl--free solution for at least 14 min inhibits the shrinkage-induced pH(i) increase by 80%. We calculated the pH(i) dependence of the Na+/H+ exchange rate in cell populations under normosmolar and hyperesmolar conditions by summing 1) the phi dependence of the total acid-extrusion rate and 2) the phi dependence of the EIPA-insensitive acid-loading rate. Shrinkage alkali shifts the phi dependence of Na+/H+ exchange by ~0.7 pH units.