The quantity and distribution of body iron normally vary little over time. This homeostasis is maintained primarily through control of intestinal iron absorption, but the mechanism of this regulation is unclear. Modern techniques for computer simulation and numerical analysis now make it possible to study the kinetics of iron absorption in vivo. We used a physiologically based mathematical model of iron metabolism to analyze tracer iron kinetics in normal and iron-deficient beagles. The model provides characteristic information about both intestinal and systemic iron exchange, thus permitting formulation of an hypothesis that may explain the regulation of iron absorption under these conditions. The results indicate that control of iron absorption is a function of independent expression of iron requirements by each tissue, including the intestinal mucosa. This hypothesis is consistent with other in vivo and in vitro observations in iron deficiency and may have implications for understanding the mechanism of the altered iron absorption in other disorders of iron metabolism.