The predominant inhibitory neurotransmitter of the brain, GABA (gamma-aminobutyric acid), activates chloride-selective ion pores integral to the receptor complex. Subunits comprising the presumed hetero-pentameric GABA channel have been cloned, but little information is available on the domains important for activation. Rat wild-type or mutated alpha 1-, beta 2- and gamma 2-subunits (designated alpha, beta and gamma) were coexpressed in Xenopus oocytes and examined electrophysiologically. We report here the identification of two separate and homologous domains of the beta-subunit, each of which contributes a tyrosine and threonine essential for activation by GABA. Conservative substitution of each of these four amino acids dramatically decreased GABA channel sensitivity to activation by GABA and the GABA agonist muscimol. These substitutions, however, did not impair activation by the barbiturate pentobarbital, indicating these two different classes of agonists activate GABA channels through distinct mechanisms. We also present evidence suggesting that the two identified domains of the beta-subunit contribute a major component of the GABA receptor.