In order to understand the renal excretion mechanisms of sulfonamides, some renal clearance studies were undertaken with two sulfonamides, sulfanilamide and sulfisoxazole, in rabbits. From the results of inhibitory experiments by iodopyracet, it was found that only sulfisoxazole was actively secreted into the tubule by p-aminohippurate mechanism. Using this method it becomes possible to calculate and separately estimate the net amounts secreted and reabsorbed. The result calculated with sulfisoxazole suggested a significant amount secreted and a higher amount reabsorbed.