This study investigated fexofenadine (FXD) transport and the inhibition of FXD transport in Caco-2 cell monolayer transwells, using rhodamine 123 (RH123) transport as a positive control. FXD transport from the basolateral (B) to apical (A) compartment was fivefold higher than A to B transport. FXD transport was linear with respect to time (up to 270 min) and concentration (up to 300 microm). Similar results were seen with the positive control RH123. Ritonavir (100 PM) and verapamil (100 microm) reduced transport of FXD and RH123 by more than 80%, whereas transport was not inhibited by 100 m indomethacin or 2 mM probenecid. This suggests predominantly P-glycoprotein (P-gp)-mediated transport as opposed to transport by multidrug resistance protein. In concentration-response experiments, FXD transport was inhibited by verapamil and ritonavir with IC50 values of 6.5 microm and 5.4 microm, respectively. Results from this in vitro study demonstrate differential transport of FXD across Caco-2 cell monolayers and inhibition of FXD transport by established P-gp inhibitors. Thefindings support the use of FXD as an index or probe compound to reflect P-gp activity in vivo.