We studied the mechanisms of K transport across an epithelium in which NaCl absorption is mediated primarily by Na/K/Cl cotransport at the apical membrane. Rubidium served as a reliable K substitute; under control conditions, both K and Rb were actively secreted. During secretion, K (Rb) enters across the basolateral membrane via the Na/K pump and exits across the apical membrane through K conductance pathways, since serosal ouabain or mucosal barium abolished K secretion, mucosal furosemide or Cl-free media blocked K secretion by interfering with access of Na to the pump, and elevated mucosal solution [K] or [Rb] depolarized the apical membrane electrical potential difference. Mucosal Ba unmasked active Rb absorption that could be blocked by mucosal furosemide. These findings illustrate active K absorption and secretion across an epithelium that comprises a single cell type in which opposing K fluxes across the apical membrane are mediated by Na/K/Cl cotransport entry and conductive K exit. The direction of transepithelial K transport is determined by the relative activities of these pathways.