Reactive airway disease predisposes patients to episodes of acute smooth muscle mediated bronchoconstriction. We have recently demonstrated for the first time the expression and function of endogenous ionotropic GABA_A channels on airway smooth muscle cells. We questioned whether endogenous GABA_A channels on airway smooth muscle could augment -agonist mediated relaxation. Guinea pig tracheal rings or human bronchial airway smooth muscle were equilibrated in organ baths with continuous digital tension recordings. Following pretreatment with or without the selective GABA_A antagonist gabazine (100µM), airway muscle was contracted with acetylcholine or -ala neurokinin A, followed by relaxation induced by cumulatively increasing concentrations of isoproterenol (1nM to 1µM) in the absence or presence of the selective GABA_A agonist muscimol (10µM to 100µM). In separate experiments, guinea pig tracheal rings were pretreated with the large conductance K_Ca channel blocker iberiotoxin (100nM) after an EC₅₀ contraction with acetylcholine but before cumulatively increasing concentrations of isoproterenol (1nM to 1µM) in the absence or presence of muscimol (100µM). GABA_A activation potentiated the relaxant effects of isoproterenol following an acetylcholine or tachykinin-induced contraction in guinea pig tracheal rings or an acetylcholine-induced contraction in human endobronchial smooth muscle. This muscimol-induced potentiation of relaxation was abolished by gabazine pretreatment, but persisted following blockade of the maxi K_Ca channel. Selective activation of endogenous GABA_A receptors significantly augments -agonist mediated relaxation of guinea pig and human airway smooth muscle which may have important therapeutic implications for patients in severe bronchospasm.