In two experiments sulphadimidine-free pigs were placed in pens previously occupied by pigs fed a diet containing 100 ppm sulphadimidine. Faeces, urine and spilled feed had been removed by scraping the surface of the pens before the new pigs were introduced. The concentration of sulphadimidine in the tissues of the medicated pigs fell below 100 ng/g within 72 hours of withdrawal of the medicated diet and in the fluids the concentration fell below 500 ng/ml within 96 hours. The concentrations in the tissues of the pigs housed in the contaminated pens exceeded 100 ng/g for up to 24 hours but then fell to acceptable concentrations; the concentration of sulphadimidine in body fluids occasionally exceeded 500 ng/ml.