Glutamine synthetase and glutamine concentrations were compared in various rat tissues, in rat liver throughout development and during regeneration, and in transplanted tumors. There was no obvious relationship between the two components in these tissues. The enzyme increased abruptly in liver during the third week of life, but was not significantly induced by hydrocortisone nor ‘repressed’ by treatment with glutamine. Eight tumors with different growth rates all contained glutamine synthetase and glutamine in low but reasonable concentrations. One highly differentiated Morris hepatoma contained more glutamine synthetase than normal liver. There was no depression of glutamine synthetase in liver throughout the growth period of the tumors examined. Levels of free glutamine in plasma and liver were, however, significantly less than normal in the tumor-bearing rats.