The aim of this experiments was to investigate the effect of Croton. Tiglii. semen water extract on the renal function, plasma renin activity, plasma levels of atrial natriuretiu peptide and aldosterone in rats. The results of this study were as follows: 1. Water balance was not changed significantly after the administration of Croton Tiglii semen water extract. 2. Urine volume decreased significantly after the administration of Croton Tiglii semen water extract $80{mu}l/200g$. 3. Urinary excretion of sodium increased significantly after the administration of Croton Tiglii semen water extract $40{mu}l/200g$, but decreased significantly after the administration of Croton Tiglii semen water extract $80{mu}l/200g$. 4. Urinary excretion of potassium decreased significantly after the administration of Croton Tiglii semen water extract $80{mu}l/200g$. 5. Urinary excretion of chloride was not changed significantly after the administration of Croton Tiglii semen water extract. 6. Free water clearance was not changed significantly after the administration of Croton Tiglii semen water extract. 7. Urinary excretion of creatinine increased significantly after the administration of Croton Tiglii semen $40{mu}l/200g$. 8. Plasma renin activity was not changed significantly after administration of Croton Tiglii semen water extract. 9. Plasma levels of atrial natriuretic peptide increased significantly after administration of Croton Tiglii semen water extract. 10. Plasma levels of aldosterone increased significantly after administration of Croton Tiglii semen $40{mu}l/200g$.