The typical western diet provides 10 mg of iron each day. However only 10% of this is absorbed in the duodenum (i.e. 1mg/day), meaning that 90% of our daily intake reaches the distal small intestine and colon. It is presumed that this excess iron is simply excreted in the faeces. However, a recent report has suggested that the proximal colon might have some iron transport capacity (Bougle et al. 2002). In the present study we have investigated this possibility by measuring iron flux across the proximal colonic mucosa in animals fed either an iron replete (control) or an iron deficient (FeD) diet. In parallel studies the effect of dietary iron on the expression of the intestinal iron transporters DMT1 and IREG1 has also been determined. All studies were carried out on male Wistar rats (250g). Rats were fed diets containing 44mg Fe/Kg (control) or <0.5mg/Kg (FeD) for 14 days prior to experimentation. For in vivo iron uptake studies, animals were anaesthetised with intraperitoneal pentobarbitone sodium (60 mg/Kg body weight) and 0.2mM ⁵⁹Fe²⁺ (complexed with 4mM ascorbate) was instilled into a tied-off segment of proximal colon. Blood samples were removed after 20, 40 and 60 min via a femoral artery cannula to determine iron transfer into the blood. Tissue iron uptake was measured at the end of each experiment by gamma counting of the colonic segments. In a separate group of animals the mucosa was isolated and used as a source of membrane protein and total RNA for analysis of iron transporter expression by Western blotting and RT-PCR respectively. Data are mean ± SEM. Statistical analysis was performed using Student’s unpaired t-test. Iron uptake was significantly increased in FeD group (162.4 ± 36.1 pmoles/mg dry wt tissue n=6) compared with control (71.4 ± 18.3 pmoles/mg dry wt tissue, p<0.05 n=6). This corresponded with a significant increase in DMT1 mRNA and protein in the FeD group. Interestingly, there was no difference in either iron efflux into the blood or in IREG1 expression in the two animal groups. These data indicate that the proximal colon has some capacity to absorb iron from the intestinal lumen. However, rather than transferring the iron into the blood for physiological utilisation, it is retained within the colonic mucosa. The physiological rationale for such a mechanism requires further investigation.