The aim of this experiment was to elucidate the effects of Saseuptang water extract on the renal function plasma renin activity and plasma levels of atrial natriuretic peptide and aldosterone in rat The results were as follows; 1. Water balance decreased significantly after the administration of Saseuptang water extract, 0.4 and 0.8ml/kg. 2. Urine volume increased significantly after the administration of Saseuptang water extract, $0.4;m{ell}/kg$. 3. Urinary excretion of chloride increased significantly after the administration of Saseupthang water extract, $0.8;m{ell}/kg$. 4. Free water clearance increased significantly after the administration of Saseuptang water extract, $0.8;m{ell}/kg$. 5. Urinary excretion of creatinine increased significantly after the administration of Saseuptang water extract, $0.8;m{ell}/kg$. 6. Plasma levels of atrial natriuretic peptide (ANP) decreased significantly after administration of Saseupthang water extract, $0.8;m{ell}/kg$. These results suggest that the changes of urine volume after the administration of Saseuptang water extracts are related to the increments of glomerular filtration rate and free water clearance, and it is suggested that the changes of renal function by which Saseuotang may related to the renin-angiotensin and atrial natriuretic peptide system.