The change of plasma renin activity is one of the most important parameters explaining the pathological physiology of the hypertension. The relation between renal renin activity and plasma renin activity has not well been documented since last decades. In an attempt to clarify the relationship a series of experiments have been done in rats. The following results were observed. 1) Renal renin activity of clamped kidney increased after silver clipping and the increments were maintained until four to five weeks of operation. 2) Renal renin activity of the untouched contralateral kidney was vulnerable to be suppressed just after clamping, and the activity disappered almost below the limitation of the radioimmunoassay sensitivity up to four weeks. 3) Plasma lenin activity was changed by the renal renin activity, but the regression coefficient from the two kidney one clip Goldblatt hypertensive rats was different from the sham-operated, or age-matched control rats. 4) Plasma renin activity of all the groups tested has the exponential curve in terms of renal renin activity. These data suggest that the renal renin activity is important to control the plasma renin activity in certain experimental condition, especially in chronic status.