To study the properties of the basolateral membrane conductance of an amphibian epithelial cell line, we have adapted the technique of apical membrane selective permeabilization (Wills, N.K., Lewis, S.A., Eaton, D.C. 1979b, J. Membrane Biol. 45:81-108). Monolayers of A6 cells cultured on permeable supports were exposed to amphotericin B. The apical membrane was effectively permeabilized, while the high electrical resistance of the tight junctions and the ionic selectivity of the basolateral membrane were preserved. Thus the transepithelial current-voltage relation reflected mostly the properties of the basolateral membrane. Under "basal" conditions, the basolateral membrane conductance was inward rectifying, highly sensitive to barium but not to quinidine. After the induction of cell swelling either by adding chloride to the apical solution or by lowering the osmolarity of the basolateral solution, a large outward-rectifying K+ conductance was observed, and addition of barium or quinidine to the basolateral side inhibited, respectively, 82.4 +/- 1.9% and 90.9 +/- 1.0% of the transepithelial current at 0 mV. Barium block was voltage dependent; the half-inhibition constant (Ki) varied from 1499 +/- 97 microM at 0 mV to 5.7 +/- 0.5 microM at -120 mV. Cell swelling induces a large quinidine-sensitive K+ conductance, changing the inward-rectifying basolateral membrane conductance observed under "basal" conditions into a conductance with outward-rectifying properties.