The effects of phenolic compounds on Na+-dependent D-glucose transport were investigated in brush border membrane vesicles isolated from rat small intestine. Screening experiments were conducted with different classes of phenolic compounds in both their native and oxidized forms. Pretreatment of vesicles with tannic acid (1 mg/ml) completely abolished the characteristic overshoot of active glucose accumulation. With chlorogenic acid (1mM), 80% of the glucose transport capacity was lost. Reductions of 30-40% were observed in vesicles treated with catechin, ferulic or caffeic acids. Treatment with gallic acid (1 mM) had little effect. Phenolic oxidation state did not exacerbate the degree of glucose transport inhibition, with the exception of catechol (1 mM), which gave maximal inhibition (86%) in its oxidized form. Gradient-independent glucose uptake was not altered, nor did phenolic treatment increase nonspecific binding of glucose to the membrane vesicles. Possible mechanisms of D-glucose transport inhibition were examined in chlorogenic acid-and tannic acid-treated vesicles. Factors such as alterations in vesicle permeability, size and leakage of transported glucose out of the vesicles were ruled out. Measurements of D-glucose uptake under conditions of Na+ equilibrium suggest that tannic and chlorogenic acids reduce glucose uptake by favoring the dissipation of the Na+ electrochemical gradient, which provides the driving force for active glucose accumulation.