Glutamine synthetase activity in several rat tissues has been measured. Liver contains the highest specific activity followed by stomach, brain, kidneys, intestine, skin, adipose tissue and striated muscle-that had the lowest specific activity both with regard to tissue, protein and DNA weight. Per unit of animal weight, liver and muscle contain similar activities. 24 Hours of fasting induced a significant decrease in liver, stomach, intestine and skin glutamine synthetase, compensated by an increase in muscle activity. During fasting, the splanchnic glutamine synthetase activity is lowered and that of peripheral tissues is increased, thus favoring a net glutamine flux from peripheral to splanchnic organs.