Intestinal extraction of circulating glutamine across the basolateral membrane is diminished in the tumor-bearing rat (TBR). This study was designed to investigate the effects of progressive malignant growth on brush border glutamine transport in order to gain further insight into the adaptive/regulatory changes in intestinal glutamine metabolism that occur in the tumor-bearing rat. Fischer 344 rats (225 ± 5 g) were implanted with fibrosarcoma cells and were studied at various time points after implantation when the tumors comprised 7%, 20%, and 29% of total body weight. Control and tumor-bearing rats were pair-fed throughout the study. Jejunal brush border membrane vesicles (BBMVs) were prepared by magnesium aggregation/differential centrifugation and transport of radioactively labeled l-glutamine, l-leucine, l-alanine, and d-glucose by BBMVs was measured using a Millipore filtration technique. BBMVs were enriched 15-fold in alkaline phosphatase, indicating brush border vesicle purity. Uptake of all substrates occurred into an osmotically active space, exhibited overshoots, and had similar 1-hr equilibrium values. The rate of glutamine uptake by BBMVs from all tumor-bearing rats was significantly greater than controls, regardless of tumor size. The increase in transport activity was not due to a change in carrier affinity but rather to an increase in maximal transport velocity. In rats with small tumors (7% of body weight), the Vmax was 431 ± 40 pmole/mg protein/10 sec compared to 259 ± 30 in control animals (P < 0.01). In marked contrast, the mean transport of alanine was diminished in BBMVs from TBR (31 ± 3 pmole/mg protein/10 sec in TBR vs 23 ± 2 in controls, P < 0.05). There was no significant difference in glucose and leucine transport between control and TBR. The results suggest that the upregulation in brush border transport that occurs in this tumor model may be specific for glutamine, indicating the high cellular priority and requirement for glutamine. Such adaptive regulation in brush border transport activity in tumor-bearing animals has not been previously described. © 1991.