In sodium depleted rats urine extracts of hydrated healthy human volunteer cause an increase in water excretion of 52% (p less than 0.005) and in sodium excretion of 45% (p less than 0.005). Cardiac output of the volumen loaded rats increases from 465 +/- 104 ml to 528 +/- 128 ml/kg-min (p=n.s.). Rb86 is used as an indicator for renal hemodynamics. Compared with controls there is an increase in blood flow through the outer medulla from 1.57 +/- 0.65 to 2.28 +/- 0.61 ml/g-min, through the inner medulla from 0.75 +/- 0.30 to 1.01 +/- 0.30 ml/g-min, and through the papillary tips from 0.85 +/- 0.04 ml to 1.21 +/- 0.25 ml/g-min. The experiments show that the diuretic/natriuretic urine extracts influence the hemodynamics of the renal medulla.However, since the significance level of the enhanced medullary blood flow is only low (p less than 0.1) it is impossible to find out the direct site of action of the urine extracts.