The processes of L-phenylalanine (Phe) uptake and efflux from the apical (AP) and basolateral (BL) sides of an intestinal epithelial cell line (Caco-2) were investigated to further characterize the mechanism of transcellular transport of this amino acid. The results indicated that the initial uptake rates of Phe were saturable with a Km of 2.7 mM for AP uptake and 0.18 mM for BL uptake. Unlike the uptake, the initial efflux rates were shown to be proportional to the intracellular concentrations of Phe. Based on these kinetic studies and determination of other characteristics (e.g., Na+ dependency) of the uptake and efflux processes, it was concluded that AP uptake, BL uptake and BL efflux were distinctly different. This suggests that either different carriers or a different combination of carriers are responsible for the transmembrane transport of this amino acid. When the results of kinetic studies of Phe uptake and efflux were used to determine the rate-limiting step in the AP-to-BL transcellular transport of this amino acid, it was concluded that the BL efflux is the rate-limiting step in the transcellular transport of Phe in the Caco-2 cell monolayers.