Activities of Na channels and Na pumps were studied in the rat cortical collecting tubule (CCT) during manipulation of the animals' mineralocorticoid status in vivo using a low-Na diet, diuretics, or administration of exogenous aldosterone. Tubules were isolated and split open to expose the luminal membrane surface. Using the whole-cell patch-clamp technique, activities of the apical Na channels and the basolateral Na pumps were measured in principal cells as the currents inhibited by amiloride (10 microM) and ouabain (1 mM), respectively. Na channel current (INa) was not measurable in CCTs from control animals on a normal diet. INa was approximately 200 pA/cell in CCTs from animals on a low-Na diet or infused with aldosterone using osmotic minipumps. Currents attributable to the Na pump (Ipump) were similar in control animals and animals on a low-Na diet. Maximal currents were approximately 35 pA/cell in both groups, and decreased with hyperpolarization of the cell membrane. In contrast, administration of exogenous aldosterone increased Ipump fourfold. Coinfusion of aldosterone and amiloride in vivo through the minipumps did not affect the induction of INa but reduced the induction of Ipump by 80%. We conclude that the induction of channel activity in this tissue is a direct action of aldosterone, whereas the induction of pump activity may be a consequence of the increased Na traffic through the epithelial cells.