The concentration of sulfadimidine was measured in the urine of pigs which were housed (over five days) in boxes where other pigs have been treated orally with sulfadimidine before. Sulfadimidine was measured in the urine of the unmedicated pigs in a concentration of up to 4 micrograms/ml. Considering these urine concentrations, violative sulfadimidine tissue residues would be expectable in the carcass after slaughter. The practice of fixing withdrawal times must be considered again to ensure that drug residues in tissues are below the MRL before slaughter also in unmedicated animals.