Two groups of non-anemic dogs, one with normal tissue iron stores and one with decreased tissue iron stores, underwent a surgical construction of two equal segments of duodenum and jejunum. A solution of 0.1 mg ferrous sulfate and 1 mg ascorbic acid was injected into the lumen of each segment. The solution for the duodenal segment contained 30 muc of iron59, for the jejunum segment 60 muc of iron55. The solutions were left in the segments for 2 1/2 hours before washout. On the 14th postoperative day, iron59 and iron55 radioactivity per ml/RBC was measured, and iron uptake by segments determined in two ways. Bone marrow, liver, and spleen were histologically assessed of iron stores. In dogs with normal iron stores, percent iron retained was slightly higher in jejunum that duodenum. In dogs with decreased iron stores percent retention was always higher in duodenum; in this group iron retained was about four times higher than in dogs with normal iron stores. This indicates that mucosal uptake of iron from lumen and mucosal transfer of iron from plasma vary inversely with body iron stores, but compared to mucosal uptake, mucosal transfer is more restricted to duodenum, more affected by body iron stores, more limited, and more crucial to regulation of iron absorption.