Microinjection of the GABA antagonist bicuculline methiodide 1-25 ng into the posterior hypothalamus of urethane-anesthetized rats evoked sympathetically-mediated increases in heart rate of up to 150 beats/min and modest increases in blood pressure which could be prevented by prior local microinjection of muscimol 50 ng. Microinjection of picrotoxin but not strychnine produced similar effects. These results suggest that a latent sympathoexcitatory mechanism in this region is tonically inhibited by endogenous GABA.