The activity of glutamine synthesis in the brain extracts of various animals were observed by means of the Conwsy's microdiffusion analysis. The brain extracts were incubated at 37℃, pH 7.2 for 25 minutes under aerobic condition. 1) The activity of glutamine synthesis in the normal rat brain extract was 1.53 μM per tube on an average. 2) In the mouse brain it averaged 0.81 μM per tube and was lower than the rat's. 3) It was 1.18 μM per tube on an average in the cat-fish brain. 4) In the human brain, even though only two cases, it was 1.80 μM per tube in average.