The control of iron absorption appears to reside in the mucosa of the gastrointestinal tract. The normal dog absorbs very little iron, but the anemic iron-depleted dog may absorb 10 to 20 times as much. This" mucosal block" of the normal dog probably is due largely to iron stores in the mucosa." Mucosal block" can be effected in the anemic iron-depleted dog by feeding of iron salts, but the degree of" mucosal block" under these conditions never reaches the high degree of" mucosal block" in the normal dog. Rapid movement of iron through the mucosa may explain the short duration of" mucosal block" due to iron feeding in the anemic iron-depleted dogs (18 to 20 hours, Fig. 1). The colon absorbs very little iron under the conditions described. The stomach and duodenum seem to be most active in its absorption. This suggests the existence of a gradient in the capacity of the gastrointestinal tract to absorb iron.