In isolated phloem segments of celery (Apium graveolens L.), a tissue highly specific for sucrose and mannitol uptake, glucose uptake occurs at very low rates and exhibits biphasic kinetics. Nonpenetrating inhibitors such as parachloromercuribenzene sulfonic acid did not inhibit glucose uptake. However, uptake was greatly inhibited by penetrating inhibitors such as N-ethylmaleimide and carbonylcyanide-m-chlorophenyl hydrazone. Carbonylcyanide-m-chlorophenyl hydrazone inhibition of uptake was reversed by washing and addition of thiol reagents to uptake solutions. Phlorizin, a competitive inhibitor of glucose caused moderate inhibition of uptake only after 3 hours of tissue exposure. Low pH, fusicoccin, and low turgor which enhance H+-sugar cotransport did not alter uptake rates. Furthermore, glucose did not induce alkalinization of the uptake media. Efflux analysis indicated that the presence of 50 millimolar unlabeled glucose in the wash media enhanced exchange of the labeled glucose across the tonoplast. Results indicate that the glucose carrier is not located at the plasmalemma but appears to be present at the membrane of an intracellular compartment, most likely the tonoplast. Carrier-mediated glucose transport in this tissue is proposed to be a facilitated diffusion.