Extracellular acidosis is cytoprotective in several models against anoxia/hypoxia and a variety of toxicants. The goal of this study was to determine the temporal relationships among toxicant exposure, the initiation of extracellular acidosis, Cl. Influx, Cl- channel inhibition and the onset of cellular death in rabbit renal proximal tubule suspensions. Extracellular acidosis was produced by adding HCl or H2SO4 to renal proximal tubule suspensions to decrease the extracellular buffer pH to 6.4 or by resuspending renal proximal tubules in a pH 6.4 buffer. The initiation of extracellular acidosis 15 min after the mitochondrial inhibitors antimycin A or carbonyl cyanide p-(trifluoromethoxy)-phenylhydrazone addition, a time point in which adenosine triphosphate levels are depleted and intracellular K+ is decreased, ameliorated lactate dehydrogenase release, a marker of necrotic cellular death. The initiation of extracellular acidosis 120 min after the addition of the toxicants tetrafluoroethyl-L-cysteine or t-butyl hydroperoxide decreased lactate dehydrogenase release 120 min later. Increased Cl- influx is an important step during the late phase of toxicant-induced cellular injury. Therefore, we determined if extracellular acidosis cytoprotection was associated with inhibition of Cl- influx and whether the Cl- channel inhibitors indanyloxyacetic acid (1.0 mM). niflumic acid (100 microM) and 5-nitro-2-(3-phenylpropylamino)-benzoic acid (100 microM) decreased Cl- influx and cellular death in renal proximal tubules exposed to antimycin A. Indeed, all three Cl- channel inhibitors significantly decreased 38Cl- influx and cellular death. In contrast, extracellular acidosis did not decrease 38Cl- influx but did prevent lactate dehydrogenase release. These results demonstrate that extracellular acidosis cytoprotection occurs during the late phase of cellular injury at a site distal to Cl- influx. Furthermore, the Cl- influx that occurs during the late phase of cellular injury and is critical for cellular swelling and lysis is sensitive to 5-nitro-2-(3-phenylpropyl-amino)-benzoic acid, niflumic acid and indanyloxyacetic acid.