AbstractIn order to define the importance of the mucosal uptake step in the intestinal regulation of iron absorption, unidirectional uptake rates of Fe3+from a nitrilotriacetic acid chelate were measured in duodenal fragments from mice using anin vitrotechnique. [57Co]‐Cyanocobalamin was used as a marker of adherent incubation medium. Uptake showed saturation kinetics over the concentration range 18–450 μM. Uptake was increased in fragments from hypoxic, dietary iron‐deficient and pregnant mice. The enhanced uptake was due to an increase inVappmax. However, the modest increase in uptake rates in pregnancy and the gross changes observed in iron‐deficiency make the hypoxic model the most convenient. The increase in uptake in hypoxic animals was located to the duodenal region and was not associated with changes in either total mucosal iron content or epithelial cell turnover. The rate of uptake of iron via the serosa did not change with hypoxia. This study implies that flux of Fe3+across the brush border is subject to adaptive regulation. The hypoxic model is suitable for investigation into the regulation of iron ho