Secondary hyperaldosteronism produced by Na+ depletion was associated with increases in salt and fluid absorption in both the small intestine and the distal colon but not in the cecum and the proximal colon. Because these changes had not been documented for the small intestine, this study focused on the regulation of this tissue. Increased NaCl and water absorption was expressed in vitro by increases in short-circuit current and transepithelial potential and in vivo by increased fluid absorption and a decreased luminal content of Na+ and water. For example, the short-circuit current in the ileum of Na+-depleted rats was 2-fold that of adrenalectomized and 1.3-fold that of adrenal-intact control animals. The short-circuit current was inhibitable 24 +/- 14% by micromolar concentrations of amiloride in Na+-deficient animals compared with 1 +/- 3% in control animals. Similarly, ileal fluid absorption in vivo was 2.3-fold higher in Na+-deficient relative to control animals. The additional fluid absorption was sensitive to 50 microM amiloride, whereas amiloride had no effect in control animals. Furthermore, the Na+ content of the chyme from the ileum of Na+-deficient animals was about half that of controls. These results suggest that mineralocorticoids can induce the amiloride-sensitive Na+ transporter in the small intestine and that this type of epithelial salt transport can become a major pathway for salt retention by the small intestine.