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Sulfadimethoxine and its three biotransformed products in man were applied to renal clearance experiments in dogs to elucidate their renal excretion mechanisms. Clearance ratio of sulfadimethoxine is extremely low compared with each biotransformed product. Of three biotransformed products, clearance ratio of sulfadimethoxine-N^1-glucuronide greatly exceeded over the clearance ratio of the other two biotransformed products. Considerable proximal tubular secretion of sulfadimethoxine-N^4-acetate was observed. On the contrary, the proximal tubular secretion of sulfadimethoxine-N^1-glucuronide and sulfadimethoxine were insufficient. Sulfadimethoxine-N^1-glucuronide considerably reduced the affinity to dog plasma protein compared with sulfadimethoxine and sulfadimethoxine-N^4-acetate.