We examined the effect of changes in lumen calcium concentration on net potassium transport by distal tubules in anesthetized rats. Tubules were perfused with a control solution that resembled interstitial fluid but lacked calcium. Experimental solutions were prepared by adding varying amounts of CaCl2 to the control solution to produce solutions with free ionic calcium concentration ([Ca2+]) of 0.2, 0.4, and 0.8 mM. In paired comparisons 0.2 mM Ca2+ did not affect net potassium transport, whereas 0.4 and 0.8 mM Ca2+ each reduced potassium secretion by approximately 30%. Unidirectional potassium fluxes using 86Rb as a tracer for potassium and transepithelial voltage (VTE) were measured to characterize further the effect of calcium on potassium transport. Presence of 0.8 mM Ca2+ in the lumen did not affect unidirectional absorptive potassium flux; therefore, the decrease in net potassium flux was accounted for entirely by a decrease in unidirectional secretory potassium flux. The lumen negative VTE measured in the late distal tubule decreased during perfusion with 0.8 mM Ca2+. These results are consistent with the hypothesis that increases in lumen (extracellular) calcium concentration in the range normally present in the distal tubule reduce net potassium secretion by decreasing the electrochemical gradient for potassium secretion.