Modification in γ-aminobutyric acid-B (GABA B ) receptors may contribute to the symptoms of some neurological and psychiatric disorders and to the clinical response to psychotherapeutics. The present study was undertaken to determine whether chronic administration of tranylcypromine (TCP), an antidepressant, and chronic stress influence GABA B receptor function in rat brain. The results indicate that TCP treatment, but not stress, increases GABA B receptor activity in the cerebral cortex, as measured by baclofen-stimulated GTPγS binding. In addition, chronic administration of TCP enhances significantly the locomotor response to a single dose of amphetamine, an effect that is abolished by restraint stress. These results indicate that although TCP administration modifies brain GABA B receptor activity, which may contribute to the antidepressant response to this agent, this effect is unrelated to the interaction of stress and TCP treatment on the locomotor response to amphetamine.