Aldosterone increases the basolateral conductance in target epithelia. The basolateral membrane of tight epithelia contains two different types of K+ conductances (GK), a "resting" and a volume-activated GK. We have studied the early effects (at 4 hours) of 500 nmol/l aldosterone on the basolateral membrane GK of A6 cells (a Xenopus laevis kidney cell line), after the permeabilization of the apical membrane with amphotericin B. In the presence of a 97 to 3 mmol/l apical to basolateral K+ gradient, the "resting", inward rectifying GK was similar in control and aldosterone treated cells. In contrast, aldosterone induced a 2-fold increase of the volume-activated quinidine sensitive GK.