Glutamine synthetase is responsible for the ATP-dependent amidation of glutamate to glutamine. In liver the enzyme is highly localized in perivenous hepatocytes; in brain the enzyme is localized in astrocytes. Portacaval anastomosis resulted in liver atrophy, hyperammonemia, and up to 90% loss of glutamine synthetase activity in liver homogenates. This effect, which appears to be irreversible, probably reflects the selective loss of perivenous hepatocytes following portacaval anastomosis. Glutamine synthetase activities in brain were unaffected by portacaval anastomosis of up to 12 weeks' duration. Enzyme activities in homogenates of skeletal muscle, on the other hand, were significantly increased at one and four weeks after shunt surgery. These effects were not the result of decreased food intake in shunted animals. These findings suggest fundamentally different regulatory mechanisms for glutamine synthetase in these tissues. Skeletal muscle may thus provide an important alternative site for ammonia detoxification after portal-systemic shunting.