Fast-growing tumors are major glutamine consumers and may alter host glutamine metabolism to benefit the tumor. Previous studies from our laboratory have demonstrated that the liver switches from an organ of glutamine balance to one of glutamine release with progressive malignant growth. However, the regulation of this change is unclear. This study examined tumor modulation of hepatic glutamine metabolism by determining the activities of glutaminase, the principle enzyme of glutamine degradation, and glutamine synthetase, the principal enzyme of glutamine synthesis. Hepatic glutamine content was also determined. Rats with a fast-growing subcutaneous fibrosarcoma (TBR) and pair-fed controls were studied at 2 and 3 weeks after tumor or sham implantation, when the tumors comprised approximately 5% and 20% of total body weight. Arterial glutamine fell with progressive tumor growth (608 ± 26 μmol/L in controls vs 494 ± 15 in TBR, p < 0.005) and was not attributable to a diminished food intake. Hepatic glutamine content was increased 45% (p < 0.01) in tumor rats at 2 weeks due in part to a 35% fall in liver glutaminase activity. At 3 weeks, glutamine synthetase activity increased by 43% (0.58 ± 0.07 μmol/mg of protein/hr in controls vs 0.83 ± 0.04 in TBR, p < 0.01) whereas glutaminase remained depressed (2.68 ± 0.12 μmol/mg of protein/hr in controls vs 2.22 ± 0.15 in TBR, p < 0.05) and glutamine content fell compared to 2 week tumor-bearing rats, consistent with accelerated hepatic glutamine release. Tumors may alter liver glutamine metabolism by modulating hepatic enzyme activity in order to provide circulating glutamine for the growing malignancy.