Aldosterone increases the basolateral conductance in target epithelia. The basolateral membrane of tight epithelia contains two different types of K⁺ conductances (G_K), a resting and a volume-activated G_K. We have studied the early effects (at 4 hours) of 500 nmol/l aldosterone on the basolateral membrane G_k 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 G_K was similar in control and aldosterone treated cells. In contrast, aldosterone induced a 2-fold increase of the volume-activated quinidine sensitive G_K.