Glutamic acid decarboxylase (GAD) enzyme activity was measured in synaptosomes prepared from the hypothalamus, the hippocampus, the striatum and the cerebral cortex of control, adrenalectomized and rats exposed to a thermal stress. Adrenalectomy caused a statistically significant decrease in the enzyme activity in the striatum, while it had no effect in the other three brain areas. On the other hand, exposure to the thermal stress resulted in a dramatic increase of GAD specific activity in all brain areas examined. This thermal stress-induced increase in enzyme activity was observed in both non-operated and adrenalectomized animals, which implies that it is not mediated by glucocorticoids.