K+ secretion is a major determinant of body K+ homeostasis. We have shown earlier that resting K+ secretion in mouse distal colon occurs via luminal BK channels. Aldo is known to enhance colonic K+ secretion. Here we investigate if Aldo stimulated K+ secretion occurs via BK channels in mouse colon. Luminal Ba2+ - and iberiotoxin(IBTX)-sensitive K+ secretion was measured in an Ussing chamber. In vitro addition of Aldo augmented Ba2+ - sensitive K+ secretion ([Delta]Isc(Ba 2+ )) 2-fold after 2 hours (threshold 10 nM). This effect was inhibited by spironolactone (1 mM) and the specific BK channel blocker IBTX (120 nM). In vivo Aldo was augmented in two separate groups of animals; one kept on a low Na+ diet for 3 weeks and another kept on a high K+ diet for 4 days. Both diets increased the resting Ba2+ -sensitive short circuit ([Delta]Isc(Ba 2+ )) 2-fold. In addition, K+ secretion up- regulated via these diets were inhibited by IBTX. In tissues from control animals and animals treated on high K+ or low Na+ diets, the K+ secretion could be further activated by luminal ionomycin (1 mM). Importantly, this ionomycin-stimulated K + secretion was absent in BK a-subunit KO mice. In summary, these data confirm that Aldo MR-dependently stimulates K+ secretion in mouse distal colon and shows that the Aldo-elevated K+ secretion occurs via an enhanced BK channel conductance.