Blockade of gamma-aminobutyric acid (GABA) in the posterior hypothalamic nucleus elicits cardiorespiratory stimulation in anesthetized rats. The present study was conducted to test the hypothesis that blockade of GABA in this cardiostimulatory area of the posterior hypothalamus in conscious animals would elicit a defense reaction characterised by a "fight or flight" response. Blockade of GABA was achieved by injecting bicuculline methiodide (BMI 1-25 ng) and picrotoxin (4-100 ng), two post-synaptic GABA antagonists and isoniazid (INH 35 and 70 micrograms), an inhibitor of the synthesis of GABA, bilaterally into the posterior hypothalamus through chronically implanted microinjection cannulae. All three drugs produced dose-dependent increases in locomotor activity, suggesting an "escape" reaction which was quantified as number of crossings and rearings. The effects of bicuculline and picrotoxin appeared immediately after the injection while those of isoniazid appeared much more slowly, attaining peak effects 24 +/- 1 min after injection. Injection of either strychnine (38 ng) into the posterior hypothalamus or bicuculline into the lateral hypothalamic area (LHA) or the dorso-medial/ventro-medial hypothalamus (DMH/VMH) did not elicit a significant increase in locomotor behavior. These results suggest that both the physiological and locomotor components of the hypothalamic defense reaction may be under tonic GABAergic inhibition in the region of the posterior hypothalamus.