['Chemotherapy doses are limited by toxicity to normal tissues. Intravenous glutamine protects liver cells from oxidant injury by increasing intracellular glutathione (GSH) content. The authors hypothesized that supplemental oral glutamine (GLN) would increase the therapeutic index of methotrexate (MTX) by improving host tolerance through changes in glutathione metabolism. The authors examined the effects of oral glutamine on tumor and host glutathione metabolism and response to methotrexate.', 'Thirty-six 300-g Fischer 344 rats were implanted with fibrosarcomas. On day 21 after implantation, rats were randomized to receive isonitrogenous isocaloric diets containing 1 g/kg/day glutamine or glycine (GLY) by gavage. On day 23 after 2 days of prefeeding, rats were randomized to one of the following four groups receiving an intraperitoneal injection of methotrexate (20 mg/kg) or saline (CON): GLN+MTX, GLY+MTX, GLN-CON, or GLY-CON. On day 24, rats were killed and studied for arterial glutamine concentration, tumor volume, kidney and gut glutaminase activity, and glutathione content (tumor, gut, heart, liver, muscle, kidney, and lung).', 'Provision of the glutamine-enriched diets to rats receiving MTX decreased tumor glutathione (2.38 +/- 0.17 in GLN+MTX vs. 2.92 +/- 0.20 in GLY+MTX, p < 0.05), whereas increasing or maintaining host glutathione stores (in gut, 2.60 +/- 0.28 in GLN+MTX vs. 1.93 +/- 0.18; in GLY+MTX, p < 0.05). Depressed glutathione levels in tumor cells increases susceptibility to chemotherapy. Significantly decreased glutathione content in tumor cells in the GLN+MTX group correlated with enhanced tumor volume loss (-0.8 +/- 1.0 mL in GLN+MTX vs. +9.5 +/- 2.0 mL in GLY+MTX, p < 0.05).'] Does glutamine enhance selectivity of chemotherapy through changes in glutathione metabolism?