The author has already reported the influence of sulfathiazole and sulfadiazine on renal function. Barnes and Kawarchi have pointed out that the chief factors influencing the renal damages are as followed (1) concentration of drug in the urine, (2) degree of acetylation of the drug, (3) urinary stasis, (4) pH of the urine, (5) temperature of the urine. In this report, the author applied sulfisoxazole to rabbits, as it is less acetylation and toxities than the other sulfa drugs, and the results were as followed. 1. The amounts of the urine was not influenced but GFR, RPF, RBF were obviously decreased by the administration of 0.05 g, RBF were obviously decreased by the administration of 0.05 g, 0.10 g, 0.20 g per 1 kg weight of rabbits for 10 days. 2. The renal insufficiency was paralleled to the amount of sulfisoxazole administrated on rabbits and the renal function was obviously obstructed.