The study of the mechanisms by which mineralocorticoids stimulate sodium absorption across distal epithelia has focused on three possible sites of action: apical sodium permeability, the basolateral Na∶K pump, and the production of high-energy substrates. Recently we developed a method for direct measurement of the current generated by the basolateral Na∶K pump of the turtle colon [15]. In the presence of mucosal amphotericin-B and serosal barium the short-circuit current across the colon can be equated with the current produced by active electrogenic exchange of sodium for potassium across the basolateral membrane. This pump current is a measure of the transport capacity of the epithelial Na∶K pump that is uncomplicated by changes in apical membrane sodium permeability. Pump currents, thus defined, were compared in control tissues and tissues treated with aldosterone in vitro. After 9h Na absorption was increased 4-fold in the aldosterone-treated tissues but the values of the pump current were identical in the two groups. This result indicates that acute stimulation of sodium absorption by aldosterone does not occur by stimulating the Na∶K pump directly.