In a series of experiments, weanling pigs (12 to 18 kg av weight) were inoculated by means of feed with scrapings and contents of dysenteric colons. Mucohemorrhagic diarrhea developed in 97% of the infected, nonmedicated pigs, and 80% of these pigs died. Prophylactic feed medication with nithiamide at dose levels of 600, 300, or 150 ppm beginning 2 days before inoculation to 28 days after inoculation prevented the development of swine dysentery and permitted performance equal to that of the noninfected nonmedicated pigs. Morbidity and mortality were reduced by medication with 100 ppm of nithiamide but these were not influenced by medication with 50 ppm. Therapeutic feed medication with nithiamide at a dose level of 150 ppm beginning the day after diarrhea first occurred was not effective. A single oral dose of nithiamide (25 mg/kg of body weight), when diarrhea first occurred in each pig, produced rapid remission of the signs of dysentery; relapses occurred 1 to 2 weeks later, however, and mortality followed. The combination of oral dosage and feed medication was highly effective in treating dysentery, preventing its recurrence, and maintaining performance.