The cell permeability of hesperetin and hesperidin, anti-allergic compounds from citrus fruits, was measured using Caco-2 monolayers. In the presence of a proton gradient, hesperetin permeated cells in the apical-to-basolateral direction at the rate (Jap-->bl) of 10.43+/-0.78 nmol/min/mg protein, which was more than 400-fold higher than that of hesperidin (0.023+/-0.008 nmol/min/mg protein). The transepithelial flux of hesperidin, both in the presence or absence of a proton gradient, was nearly the same and was inversely correlated with the transepithelial electrical resistance (TER), indicating that the transport of hesperidin was mainly via paracellular diffusion. In contrast, the transepithelial flux of hesperetin was almost constant irrespective of the TER. Apically loaded NaN3 or carbonyl cyanide m-chlorophenylhydrazone (CCCP) decreased the Jap-->bl of hesperetin, in the presence of proton gradient, by one-half. In the absence of a proton gradient, both Jap-->bl and Jbl-->ap of hesperetin were almost the same (5.75+/-0.40 and 5.16+/-0.73 nmol/min/mg protein). Jbl-->ap of hesperetin in the presence of a proton gradient was lower than Jbl-->ap in the absence of a proton gradient. Furthermore, Jbl-->ap in the presence of a proton gradient remarkably increased upon addition of NaN3 specifically to the apical side. These results indicate that hesperetin is absorbed by transcellular transport, which occurs mainly via proton-coupled active transport, and passive diffusion. Thus, hesperetin is efficiently absorbed from the intestine, whereas hesperidin is poorly transported via the paracellular pathway and its transport is highly dependent on conversion to hesperetin via the hydrolytic action of microflora. We have given novel insight to the absorption characteristics of hesperetin, that is proton-coupled and energy-dependent polarized transport.