The addition of cobalt to oral test doses of radioiron decreases iron absorption. Studies were performed in rats to determine the site and mechanisms of this inhibition. Significantly less radioiron was observed in both the carcass and small intestine of rats fed cobalt with test doses of radioiron than in animals receiving iron alone. This indicates that cobalt diminished the mucosal uptake of iron from the intestinal lumen. Iron absorption studies in rats receiving various doses of iron with and without added cobalt suggested that cobalt was capable of saturating a common pathway in intestinal absorptive cells for the absorption of both metals.