By in vitro and in vivo bioassay methods, activity of plasma renin was studied in patients with acute tubular necrosis, polyarteritis, systemic lupus erythematosus, or the terminal stage of chronic renal disease.Plasma renin activity was increased during the oliguric phase of acute tubular necrosis, polyarteritis, or systemic lupus erythematosus; it was not increased during the diuretic phase of acute tubular necrosis and in the terminal stage of chronic renal disease.Acute reversible hypertension, observed in some patients during the oliguric phase of acute tubular necrosis, vas associated with a marked increase in plasma renin. A similar association was observed in hypertensive patients with polyarteritis or systemic lupus erythematosus, but not in hypertensive patients with the terminal stage of chronic renal disease. These data suggest that the renin mechanism may participate in the acute hypertension of acute tubular necrosis, polyarteritis, and systemic lupus erythematosus, but not in chronic hypertension in the present study.