Split renal function tests by means of single catheter techn i que were performed under urea diuresis and ADH-urea loading. The subjects studied consisted of 7 patients with unilateral atrophic pyelonephritis, of whom 2 with hypertension and 5 without hypertension (2 males and 5 females with the mean age of 38.7 years old), 6 patients with unilateral main artery obstruction male and 5 females with the mean age of 35.0 years old) and 10 normal persons as control (7 males and 3 females with the mean age of 30.2 years old). The results obtained are summarized as follows. 1) In t h e control group, the per cent comparison of the left and right values was more than 80% for all of Uf, UPAH, USTS, NNa, Uosm, Ucreat, CPAH, CSTS, Cosm, CH20, CNa, Uf / GFR x 100, Cosm/GFR x 100, Clizo/GFR x 100, CNa/GFR x 100 and UNa/Uf. Particularly, the result of Uosm showed the least difference between the both sides, with the mean comparative percentage of 98.0%. 2) The va l u es of Uf, CPAH (RPF), CSTS (GFR), Cosm, CNa and UNa x Uf were lower in the affected side in both of atrophic pyelonephritis and renal artery obstruction. The differences between the both sides were larger in the order of CPAH (value being 29.9% of the healthy side), CSTS and Uf in atrophic pyelonephritis, and Uf (23,8%), CSTS and CPAH in renal artery obstruction. 3) The values of UPAH, USTS and Uosm were higher in the affected side than in the healtly side in renal artery obstruction. More than twice concentrations were noted for UPAH and USTS in all except one case. On the other hand, these values were lower than two thirds in the affected side in atrophic pyelonephritis. Such distinct difference as in UPAH and USTS was not noted in Uosm. 4) UNa w as found to be low in the affected side in renal artery obstruction but this was not consistent in atrophic pyelonephritis. 5) The values of CN a/GFR x100 and Uf /GFR x 100 were higher in the affected side in atrophic pyelonephritis and were lower in renal artery obstruction. This suggested increased reabsorption of Na and water for unit of nephron. 6) The value of CH2o/GFR x 100 be f o re and after ADH loading showed inverse patterns between renal artery obstruction and atrophic pyelonephritis. In the former the value was higher in the healthy side under diuresis before ADH loading, while it was higher in the affected side under anti-diuresis following ADH loading. In atrophic pyelonephritis, the adverse results were noted. This suggested that a unit of nephron in the affected kidney has tendency of increased water reabsorption in renal artery obstruction while it has tendency of increased water excretion in atrophic pyelonephritis. 7) The value of (Cu2o o ff ADH—Cu2o on ADH) was interpreted to represent the width between dilution and concentration powers. In both diseases, the value was very small in the affected kidney. 8) T R FR was positive in 5 cases of renal artery stenosis and in only 2 cases of atrophic pyelonephritis.