The ureters were ligated in 6 dogs, and phthalylsulfathiazole was administered orally in doses of 0.5-0.75 g./kg./day for 3-6 days pre-operatively and post-operatively until death. Two of these animals showed, at autopsy, an increased vascularity of the fascial sheath covering the perirenal fat, 5 animals showed perirenal sheath hemorrhage, and 3 animals had fresh free blood in the abdominal cavity. The ureters were ligated in 6 control dogs, and they received no phthalylsulfathiazole. Four of these animals showed at autopsy an increased vascularity of the perirenal sheath. None of the animals showed perirenal hemorrhage. Both ureter ligation and phthalylsulfathiazole admn. are necessary for production of the perirenal lesions. It appears that the perirenal hemorrhage is due to vascular changes since no abnormalities in prothrombin time, bleeding, clotting, and clot retraction time, red blood cell counts or platelet counts were demonstrated. The avg. total blood sulfathiazole concn. at the time of death in the test animals was 2.9 mg. %.