Abstract— dl‐Allylglycine, a potent inhibitor of glutamate decarboxylase in vivo when given intraperitoneally, causes a marked decrease in brain GABA concentration and at the same time a dramatic increase in l‐ornithine decarboxylase activity and a simultaneous decrease in S‐adenosyl‐l‐methionine decarboxylase activity followed by putrescine accumulation. It does not, however, alter the degree of GABA formation from putrescine. The timing of the recovery of glutamate decarboxylase activity after the injection of dl‐allylglycine is concomitant with that of the GABA concentration, indicating that it is probably glutamate decarboxylase that is solely responsible for making up the GABA deficit caused by dl‐allylglycine, and that the changes in polyamine metabolism are associated in some indirect way with the recovery process.