The velocity of transport and shape of a pulse of radioactive indole-3-acetic acid (IAA) applied to a section of maize (Zea mays L.) coleoptile depends strongly on the concentration of nonradioactive auxin in which the section has been incubated before, during, and after the radioactive pulse. A pulse of [3H]IAA disperses slowly in sections incubated in buffer (pH 6) alone; but when 0.5-5 laM IAA is included, the pulse achieves its maximum velocity of about 2 cm h -1. At still higher IAA concentrations in the medium, a transition occurs from a discrete, downwardly migrating pulse to a slowly advancing profile. Specificity of IAA in the latter effect is indicated by the observation that benzoic acid, which is taken up to an even greater extent than IAA, does not inhibit movement of [3H]IAA. These results fully substantiate the hypothesis that auxin transport consists of a saturable flux of auxin anions (A-) in parallel with a nonsaturable flux of undissociated tAA (HA), with both fluxes operating down their respective concentration gradients. When the anion site saturates, the movement of [3H]IAA is nonpolar and dominated by the diffusion of HA. Saturating polar transport also results in greater cellular accumulation of auxin, indicating that the same site mediates the cellular efflux of A-. The transport inhibitors napthylphthalamic acid and 2,3,5-triiodobenzoic acid specifically block the polar A-component of auxin transport without affecting the nonsaturable component. The transport can be saturated at any point during its passage through the section, indicating that the carriers are distributed throughout the tissue, most likely in the plasmalemma of each cell.