Evidences from experiments with stem sections of sunflower seedlings suggest that the transport of auxin may be limited by a restricted pool size of transportable auxin and restrictions in the availability of transport sites. A steady state of transport is observed over a range of lengths of stem sections, and over a wide range of auxin contents. The capacity of the sections to transport a pulse of auxin declines with aging after cutting, 50% decline occurring at about 10(+) hours; the transportability of a pulse of auxin declines rapidly after the completion of uptake, 50% decline occurring at about 1 hour. A chase treatment with unlabeled auxin does not alter transport, but a pretreatment with auxin depressed subsequent transport for about 1 hour. In depleted tissues such pretreatment is not inhibitory but rather is promotive of transport. The interpretation offered is that transport is limited by the pool size and transport sites, and roles for these factors are suggested in relation to the auxin transport gradient and the tropistic responses.