The effect of iron on manganese transport through, and its retention in, the duodenal wall was studied in 5-week-old female albino rats. Animals were fed only cow's milk supplemented with different doses of ferrous sulphate (0.60-19.0 mg Fe/100 ml) for 3 days before killing. Manganese transport was studied in vitro by the method of 'everted gut sac.' In all rats fed milk supplemented with iron the manganese transport through, and its retention within, the intestinal wall was signifcantly lower than when no iron was given (0.001 greater than p less than 0.001). The suppression of both the transfer and intestinal retention of managanese suggests a saturation effect produced by iron which levels off above 5 mg Fe/100 ml milk.