Microinjections into the mesencephalic central gray (CG) or the medial hypothalamus (MH) of three drugs (L-allylglycine, Semicarbazide or 4,5 dihydroxy-isophtalic acid) known to block glutamic acid decarboxylase (GAD) produced a dose-dependent behavioral activation accompanied by jumps. These effects are qualitatively similar to those produced by microinjections of SR 95103 (a GABA-A receptor antagonist) at the same site. These findings suggest that, at both the level of the CG and the MH, gamma-aminobutyric acid (GABA) tonically inhibits a neuronal substrate involved in the generation of flight reactions.