1. A radiochemical assay for glutamine synthetase has been developed in which an ATP-regenerating system is incorporated to prevent accumulation of inhibitory amounts of ADP. It is particularly suitable for assay of the enzyme in crude tissue extracts containing high adenosine triphosphatase activity. 2. A survey of the distribution of the enzyme in tissues from normal male rats showed that activity is present in liver, brain cortex, kidney cortex, spleen, testis and retina. 3. The Km of the enzyme for l-glutamate is approx. 1.5×10−2m.
