The early effects (1-4 h) of aldosterone on membrane conductance were studied in the TBM cells, an epithelial cell line derived from the toad urinary bladder. Aldosterone stimulated Na+ transport (measured as short-circuit current) and induced parallel increases of the amiloride-sensitive apical membrane Na+ conductance and the basolateral membrane conductance. In another set of experiments, we prevented the aldosterone-induced increase of the Na+ transport by adding amiloride to the apical solution in concentrations sufficient to reduce short-circuit current to values similar to that of matched control. The effect of aldosterone on the basolateral conductance was not inhibited. This effect was essentially due to a change of the barium-sensitive K(+)-specific conductance. We also observed that aldosterone prevented a time-dependent decline of the paracellular resistance observed in control preparations. Aldosterone induces an early increase of the basolateral membrane conductance, an effect that is not secondary to the increase of the Na+ load to the cell.