Strategies are sought to reduce pathogenic Escherichia coli concentrations in food animals. Because E. coli possess respiratory nitrate reductase activity, which also reduces chlorate to cytotoxic chlorite, we tested and found that oral sodium chlorate administration reduced gut concentrations of E. coli O157:H7 in experimentally infected pigs and wildtype E. coli concentrations in nonchallenged pigs. Mean +/- S.E. concentrations (log10 CFU/g) of E. coli O157:H7 in ileal, cecal, colonic and rectal contents from placebo-treated pigs were 4.03 +/- 0.66, 3.82 +/- 0.24, 4.42 +/- 0.25 and 4.03 +/- 0.16, respectively. In contrast, E. coli O157:H7 concentrations were reduced (P < 0.05) in ileal (1.56 +/- 0.22) cecal (2.65 +/- 0.38), colonic (3.05 +/- 0.38) and rectal (3.00 +/- 0.29) contents from pigs orally administered three successive (8 h apart) 10-ml doses of 100 mM chlorate. Wildtype E. coli concentrations in gut contents of non-E. coli O157:H7-challenged pigs likewise treated with chlorate were reduced by 1.1 to 4.5 log10 units compared to concentrations in placebo-treated pigs, which exceeded 6.0 log10 CFU/g. As before, the reductions were greater in anterior regions of the gut than regions more caudal. Similar treatment of E. coli O157:H7-challenged pigs with 200 mM chlorate caused reductions in gut concentrations of E. coli O157:H7; however, the reductions were not necessarily greater than those achieved with the 100 mM chlorate treatment.