Time- and dose-response studies were carried out on the effects of the monoamine oxidase-inhibiting antidepressant and antipanic drug phenelzine on GABA levels in rat whole brain. At a dose of 15 mg/kg i.p., phenelzine significantly elevated GABA levels at all time intervals studied (1, 2, 4, 8, 16, and 24 h). A further investigation indicated that this was a dose-dependent effect. The possible importance of GABA in the etiology and pharmacotherapy of depression and panic disorder is discussed.