There is evidence that iron stores alter iron absorption to meet body iron needs. In this study, quantitative aspects of this regulation of iron balance are examined. Two male subjects, one with low and one with average iron stores, were given additional dietary iron over 500 days. Changes in iron stores were monitored by plasma ferritin measurements, and the ferritin estimate of stores was validated at the end of the study by bleeding to the point of iron deficiency. The subject with low iron stores increased his ferritin by 39 micrograms/L or 0.8 mg/day, whereas the subject with average iron stores had no significant change. It is concluded that the mechanism by which stores regulate food iron absorption has a capacity of about 1 mg/day, sufficient to meet usual physiological needs, with the exception of pregnancy, as well as the ability to resist increase in stores above normal despite an iron rich diet.