In this paper we describe the first observation of GABA inhibition in an experimental model of asthma in vivo. Guinea-pigs were actively sensitized with ovalbumin i.p. and 20 days later the Herxheimer microshock was performed. GABA and (-)-baclofen injected 20 min previously significantly prevented the development of microshock. Therefore GABAergic drugs appear to modulate in vivo anaphylactic reaction. The value of this observation with regard to the physiopathology and therapy of asthma remains to be elucidated.