Plasma renin activity and renin substrate were measured in nine groups of rats which were maintained for 7 wk on diets in which the proportions of sodium and potassium were varied. Balance data indicated that the highest dietary intake of potassium employed (92 mEq K(+)/100 g food) consistently induced sodium depletion. With less consistency, the highest sodium intake employed (52 mEq Na(+)/100 g food) tended to induce potassium depletion.In accordance with previous reports, sodium deprivation induced significant increases in plasma renin activity. But the present results indicated that changes in potassium intake exerted a highly significant modulating influence on this characteristic response. The results describe an inverse relationship between potassium administration and the concurrent level of plasma renin activity. The highest serum renin levels of all occurred in the potassium-depleted animals and the usual renin response to sodium deprivation was virtually abolished in the presence of a high potassium diet. Neither the suppressing effect of K(+) administration nor the stimulating effect of K(+) depletion on plasma renin activity could be explained in terms of any predicted changes in aldosterone secretion or observed changes in sodium balance. Therefore, the effect seems to be mediated by a direct influence of potassium ions on renal renin secretion, perhaps via induced changes in sodium load to the macula densa.These studies point to an important role for potassium in the regulation of renin secretion. The results in turn raise the possibility that renin secretion per se may be importantly involved in effecting potassium conservation and potassium elimination. The means by which these interactions are finally mediated remain to be clarified.