Abstract 1. 1. Measurement of glutamic acid decarboxylase in various brain areas of 7 mammalian species (guinea-pigs, rats, gerbils, cats, sheep, harbour seals, and humans), using a micro-radiometric method, indicated generally similar distribution patterns. 2. 2. The order of activity is higher than indicated by calculations from reported GABA levels and turnover rates, suggesting either excess enzyme capacity or the existence of a GABA compartment with high turnover rate. 3. 3. The human data obtained are in accord with those previously reported and indicate possible association of high activity both with the extrapyramidal system and with areas involved in seizure phenomena.