The effect of some flavonoids on the K(+)-dependent and K(+)-independent leucine uptake into brush border membrane vesicles from Bombyx mori larval midgut was investigated. Among the compounds tested, cyclochampedol, recently purified from Artocarpus champeden, was able to inhibit in micromolar range the leucine transport. The inhibition occurred both in the absence and in the presence of potassium and was not affected by leucine concentration. The apparent Ki was 0.25 mM. Cyclochampedol represents the first non-competitive inhibitor of an amino acid transport system in Lepidoptera. The relevance of this result is discussed.