L-Glutamine, a major energy substrate for intestinal epithelial cells, can be extracted from intraluminal contents across the brush-border membrane and from arterial blood via the basolateral membrane. The purpose of the present study was to characterize glutamine transport by the basolateral membrane of rabbit epithelial cells. Transport of glutamine by isolated basolateral-membrane vesicles was mediated by both Na(+)-dependent and Na(+)-independent carriers. Tests were performed to distinguish glutamine uptake by likely transport agencies, including Systems A, ASC, N, IMINO, NBB, L and asc. The Na(+)-dependent glutamine uptake was strongly inhibited by an excess of 2-(methylamino)isobutyric acid (MeAIB), and glutamine was equally effective in inhibiting MeAIB transport. The reciprocal inhibition analysis, as well as a sensitivity to increased H+ concentration, indicates that Na(+)-dependent glutamine transport across the basolateral membrane is mediated by System A. The saturable Na(+)-independent glutamine transport was markedly inhibited by 2-aminobicyclo-[2,2,1]-heptane-2-carboxylic acid ('BCH') and insensitive to changes in assay pH, suggesting uptake via System L rather than System asc. The presence of a Na(+)-dependent carrier to mediate active transport of glutamine across the basolateral membrane is probably essential to ensure a continuous supply of this vital substrate to the enterocyte in the post-absorptive state.