The uptake of zinc as acexamic acid salt in the rat gastrointestinal tract, using an in situ static technique, was studied. Our aim was to investigate an absorption window for zinc and the uptake kinetics in the colon. To detect selectivity phenomena in zinc absorption, buffered saline solutions of zinc (50 micrograms/ ml) were perfused in stomach, whole colon and three 33-cm fractions of the small intestine (proximal, middle and distal segments). To characterize zinc uptake kinetics in whole colon, five different zinc concentrations (5, 25, 50, 150 y 250 micrograms/ml) were assayed. Zinc secreted into the gastrointestinal tract during the experiments was deducted from the uptake. Zinc secretion was characterized as an apparent zero-order process for all the studied segments (mean secretion rate = 0.10 +/- 0.03 microgram/(ml x min)). The stomach exhibited little ability to absorb zinc (apparent first order rate constant = 0.17 +/- 0.07 h-1), whereas the highest transport rates were found in the last two thirds of the small intestine and colon (first order constants: 0.66 +/- 0.13 h-1, 1.00 +/- 0.06 h-1, 0.97 +/- 0.14 h-1, 0.96 +/- 0.19 h-1 for proximal, middle, distal and colon segments, respectively). Zinc uptake in the colon was characterized by means of a Michaelis-Menten and first-order combined kinetics, with the following parameters: Vm = 0.36 +/- 0.02 microgram/(ml x min), Km = 18.01 +/- 0.40 microgram /ml and Ka = 0.40 +/- 0.01 h-1. Zinc is preferably absorbed in the middle and distal parts of the rat gastrointestinal tract. In the colon a saturable mechanism may be involved in apparent absorption.