1  Facultad de Medicina, Instituto de Ciencias Biomédicas, Universidad de Chile, Santiago 6530499; and 2  Centro de Estudios Científicos de Santiago, Santiago 6760470, Chile Organic osmolyte and halide permeability pathways activated in epithelial HeLa cells by cell swelling were studied by radiotracer efflux techniques and single-cell volume measurements. The replacement of extracellular Cl by anions that are more permeant through the volume-activated Cl channel, as indicated by electrophysiological measurements, significantly decreased taurine efflux. In the presence of less-permeant anions, an increase in taurine efflux was observed. Simultaneous measurement of the 125 I, used as a tracer for Cl , and 3 Htaurine efflux showed that the time courses for the two effluxes differed. In Cl -rich medium the increase in I efflux was transient, whereas that for taurine was sustained. Osmosensitive Cl conductance, assessed by measuring changes in cell volume, increased rapidly after hypotonic shock. The influx of taurine was able to counteract Cl conductance-dependent cell shrinkage but only ~4 min after triggering cell swelling. This taurine-induced effect was blocked by DIDS. Differences in anion sensitivity, the time course of activation, and sensitivity to DIDS suggest that the main cell swelling-activated permeability pathways for taurine and Cl are separate. cell volume regulation; taurine efflux; iodide efflux; organic osmolyte