Microcytic anemia (mk) mice and Belgrade (b) rats are severely iron deficient because of impaired intestinal iron absorption and defective iron metabolism in peripheral tissues. Both animals carry a glycine to arginine substitution at position 185 in the iron transporter known as Nramp2/DMT1 (divalent metal transporter 1). DMT1 messenger RNA (mRNA) and protein expression has been examined in the gastrointestinal tract ofmk mice. Northern blot analysis indicates that, by comparison to mk/+ heterozygotes, mk/mkhomozygotes show a dramatic increase in the level of DMT1 mRNA in the duodenum. This increase in RNA expression is paralleled by a concomitant increase of the 100-kd DMT1 isoform I protein expression in the duodenum. Immunohistochemical analyses show that, as for normal mice on a low-iron diet, DMT1 expression in enterocytes ofmk/mk mice is restricted to the duodenum. However, and in contrast to normal enterocytes, little if any expression of DMT1 is seen at the apical membrane in mk/mk mice. These results suggest that the G185R mutation, which was shown to impair the transport properties of DMT1, also affects the membrane targeting of the protein in mk/mk enterocytes. This loss of function of DMT1 is paralleled by a dramatic increase in expression of the defective protein in mk/mk mice. This is consistent with a feedback regulation of DMT1 expression by iron stores.