Zinc absorption by intestines of rats fed either zinc-deficient or zinc-adequate diets was measured by simultaneous luminal and vascular perfusion. The rate of absorption increased steadily for the first 30-40 min of perfusion over the entire lumen zinc concentration range examined (8-234 microM). During the last 10 min of perfusion the absorption rate did not change appreciably with time (steady state), and absorption by intestines from the zinc-depleted rats showed evidence of both carrier-mediated (saturable) and nonmediated (nonsaturable) components. The former had a Km of 55 microM and a Jmax of 3.3 nmol X min-1. In contrast, the absorption rate during the last 10 min of the perfusion period was a sigmoidal function of the luminal zinc concentration with intestines from zinc-adequate rats. The cytosol 65Zn concentration increased proportionately with increased luminal 65Zn concentration but was significantly greater in intestines from zinc-depleted rats. Both high-molecular-weight species and metallothionein accounted for this increase in cytosolic 65Zn. An increase in the vascular bed zinc concentration increased the amount of zinc transferred to the mucosal cytosol but did not increase the amount transferred to the lumen. These results suggest a second mechanism of absorption may be induced during a dietary zinc depletion that is carrier mediated. Moreover, while zinc is transferred in the vascular-to-mucosal direction from the vascular supply, little actually enters the lumen.