Effect of the continuous administration of pyridinolcarbamate (Pc) on the renal excretion of sulfonamides was investigated in rabbits. The continuous Pc administration significantly decreased the renal excretion of N⁴-acetylsulfanilamide and N⁴-acetyl-sulfadimethoxine, which are major metabolites of sulfanilamide and sulfadimethoxine. On the other hand, continuous Pc administration did not affect the renal excretion of unchanged sulfanilamide and unchanged sulfadimethoxine. It has been known that N⁴-acetylsulfanilamide and N⁴-acetylsulfadimethoxine are excreted by both the glomerular filtration and tubular secretion, but unchanged sulfanilamide and unchanged sulfadimethoxine are excreted by only the glomerular filtration. Therefore, the present experimental results suggest that the continuous Pc administration inhibits the tubular secretion of N⁴-acetylsulfanilamide and N⁴-acetylsulfadimethoxine.