1. Potassium salts increase sodium excretion in humans. To define the role of the potassium ion in this effect, we compared the effects of equimolar single oral loads of 100 mmol of NaCl and KCl on renal electrolyte excretion in seven healthy subjects. In a second group (n = 7), we infused equimolar loads of NaCl or KCl (0.75 mmol/kg in 2 h). 2. In both experiments the KCl load quickly increased plasma potassium and aldosterone concentrations and potassium and sodium excretion to a maximum by 2 h after the load, whereas the NaCl load had no such effect. 3. In a third group (n = 7) we compared the effects of single oral loads of KCl and KHCO 3 (1 mmol/kg), to assess the role of the anion in the natriuretic effect of potassium salts. 4. KCl and KHCO 3 transiently stimulated urinary excretion of potassium and sodium in an identical manner. 5. We also followed the changes in acid excretion over time. Whereas both KCl and KHCO 3 loading decreased acid excretion, this effect was greater after KHCO 3 loading. Interestingly, acid excretion did not decrease further after the first collection hour after the potassium load, although the plasma potassium concentration was still increasing. 6. From these data we conclude (1) that increased excretion of sodium, potassium and chloride and decreased excretion of protons after administration of potassium salts are the specific effects of the potassium component; (2) that potassium also appears to have secondary, indirect effects on proton excretion, the mechanism of which remains to be clarified.