GAMMA-AMINOBUTYRIC ACID was found, by Roberts et al,1 Awapara et al,2 and proved by Udenfriend,3 to be present in considerable amounts in the mammalian brain. The significance of this discovery escaped notice until Florey4 found that an extract from the mammalian central nervous system inhibited the spontaneous activity of the stretch receptor neuron of the crayfish. He also demonstrated the ability of this extract to block the effect of acetylcholine on this receptor neuron. He called this material "Factor I." Bazemore et al5 subsequently identified "Factor I" as being primarily composed of gamma-aminobutyric acid. Interest in this substance really perked up, however, with the report of Hayashi et al6 that gamma-amino-hydroxybutyric acid inhibited, in dogs, convulsions induced by electrical or chemical stimulation; Killam and Bain's7 findings that semicarbazide induced convulsions in rats are associated with a decrease in brain gamma-aminobutyric acid concentration.