The aim of this experiments was to investigate the effect of Semen Plantaginis extract on. the renal function, plasma renin activity, plasma levels of atrial natriuretic peptide and aldosterone in rats. The results obtained were as follows: 1. Water balance decreased significantly after the administration of Semen Plantaginis extract. 2. Urine volume increased significantly after the administration of Semen Plantaginis extract, 0.1ml/200g. 3. Urinary excretion of sodium decreased significantly after the administration of Semen Plantaginis extract, 0.1 ml/200 g, 2week. 4. Urinary excretion of potassium decreased significantly after the administration of Semen Plantaginis extract, 0.1 ml/200 g, 2week. 5. Urinary excretion of chloride decreased significantly after the administration of Semen Plantaginis extract. 6. Free water clearance increased significantly after the administration of Semen Plantaginis extract 0.1 ml/200 g, 2 week and 0.2 ml/200 g. 7. Urinary excretion of creatinine increased significantly after the administration of Semen Plantaginis extract. 8. Plasma renin activity decreased significantly after the administration of Semen Plantaginis extract, 0.2 ml/200 g. 9. Plasma levels of aldosterone decreased significantly after the administration of Semen Plantaginis extract. 10. Plasma levels of atrial natriuretic peptide (ANP) decreased significantly after the administration of Semen Plantaginis extract.