To more clearly define the role of the transepithelial electrical potential difference (V m→s), potassium permeability, and sodium-potassium pump rate in transcellular potassium transport by isolated turtle colon, we measured transmural potassium fluxes under open-circuit conditions in the presence and absence of putative blockers of potassium transport: amiloride and barium. The results were consistent with the notion that V m→s is a major determinant of cellular potassium secretion, whereas active potassium absorption is insensitive to changes in V m→s. These observations suggest that "coupling" between colonic sodium absorption and potassium secretion in vivo could be due primarily to the effect of the lumen negative V m→s on transcellular secretory potassium flow. Amiloride-induced inhibition of potassium secretion appeared to be due to the reductions in V m→s and sodium-potassium pump rate that accompanied the inhibition of active sodium absorption.