To clarify the mechanism by which potassium (KCl) protects against the blood pressure rising action of sodium (NaCl), we studied the effects of KCl loading in patients with idiopathic hypertension who, after a period of NaCl restriction, partook of a high NaCl diet. Eleven patients who had taken the KCl supplement (96 mEq/day) during the high NaCl period showed lesser mean blood pressure (MAP) rise with changes in NaCl intake from 25 to 250 mEq/day than 12 patients who had not taken the KCI supplement (p< 0.001). With a high NaCl diet, the KCl-supplemented patients retained less NaCl, gained less weight, and showed a lesser increase in plasma volume and cardiac output than the non-KCl-supplemented ones. Overall, the increase in blood pressure levels during the high Na diet correlated directly either with changes in plasma volume (p< 0.05) or with changes in cardiac output (p< 0.01). The results suggest that KCI may prevent a rise in blood pressure with NaCl loads in hypertensive patients by attenuating the increase in cardiac output, mainly as a result of the natriuresis. Furthermore, plasma norepinephrine was measured to estimate the sympathetic activity, since the sympathetic nervous system is known to control urinary NaCl excretion. From the low NaCl diet to Day 3 of the high NaCl diet, plasma norepinephrine was significantly (p< 0.01) decreased in the KCl-supplemented patients, whereas it remained unchanged in the non-KCl-supplemented ones. Concomitantly, urinary Na excretion was significantly greater in the early period of NaCl loading in the KCl-supplemented group as compared to the other group. Taken together, these results suggest that lower levels of norepinephrine measured in the plasma of the KCl-supplemented patients in the early NaCI-loading phases of the study are indicative of reduced adrenergic neural activity, which might be involved not only in the attenuation of increased cardiac output, but also in the responses of the kidney to shift the pressure-natriuresis relationship toward normal, leading to the natriuresis.