The effects of dietary phenolic compounds on intestinal sucrase were investigated in brush border membrane vesicles purified from rat small intestine. Screening experiments were conducted with different classes of phenolic compounds in both oxidized and native forms. The most potent inhibitor was native tannic acid at 0.1 mg/ml, resulting in an 80% loss of activity. Oxidized tannic acid had no effect. Significant decreases were also observed in vesicles treated with 0.1 mg/ml of catechol or epicatechin, yielding activity losses of 30-50%, regardless of oxidation state. With gallic acid, maximal (40%) inhibition occurred only in the oxidized form. Other phenolic compounds, such as ferulic, p-coumaric and caffeic acids, tended to be slightly inhibitory, while no inhibition was observed with vanillin or chlorogenic acid at the concentrations tested. These results confirm the enzyme inhibitory action of tannic acid, a polyphenolic compound, and also demonstrate that some individual dietary phenolic monomers have the potential to modulate enzyme activity in a brush border membrane vesicle model system.