Purpose. To evaluate the effects of fresh lime, lemon, grapefruit, and pummelo juices on the transport of digoxin, a P- glycoprotein (P- gp) substrate, in Caco- 2 cell monolayers.Methods. Bidirectional [H-3]- digoxin fluxes across confluent Caco- 2 cell monolayers were determined in 0- 50% fruit juices at pH 7.4. Verapamil HCl (100 muM) served as positive control. Juice toxicity was evaluated by the 3-( 4,5 dimethylthiazolyl- 2)- 2,5- diphenyltetrazolium bromide assay.Results. Apical- to- basal (A- to- B) digoxin flux was enhanced by 50% fruit juice in the order of lemon > lime > pummelo > grapefruit. The four fruit juices could be divided into two groups based on their effects on transepithelial electrical resistance (TEER), viability, and digoxin transport activity of the Caco- 2 cells. Grapefruit and pummelo juices produced similar digoxin transport profiles that were characteristic of those observed with P- gp inhibitors. Both juices decreased net digoxin efflux by 1.2 U per 10% increase in juice concentration and had a propensity to increase cellular TEER at high concentrations (> 30%). However, cellular TEER and viability decreased with increasing concentration of lime and lemon juices. Both juices also produced similar digoxin transport profiles, the A- to- B and B- to- A digoxin P-app increasing with increasing juice concentration above 5%. Net digoxin efflux was 30% of control value and relatively independent of juice concentration. These results paralleled the groupings of the four fruits according to their prominent flavonoid pattern and taxonomy.Conclusion. The effects of lime, lemon, grapefruit, and pummelo juices on the TEER, viability, and digoxin transport activity of the Caco- 2 cells appeared to be dependent on the dominant flavonoid pattern and taxonomy of the citrus fruits.