Three selective B2-adrenergic stimulants were studied for their effect on the resting tone and on reversing acetylcholine(ACh)-induced bronchoconstriction in the intra-pulmonary airways of rats in vitro. Neither l-(3,5-dihydroxyphenyl)-2-[(1l-[4-hydroxybenzyl]-ethyl)-amino]-ethanol-hydrobromide (Fenoterol) nor 2-tert-butyIamino-1-(3,5-dihy-droxyphenyl)-ethanol-sulphate (Terbutaline) had an effect on the resting tone. 4-Amino-3,5-dichlor-α-[(tert-butylamino)-methyl]-benzylalcohol-hydrochloride (NAB 365) caused a significant increase in the bronchial luminal diameter from control in the resting state at concentrations above 10-5 g/ml. Only NAB 365 was effective in reversing ACh-induced bronchoconstriction in concentrations above 10-6 g/ml. Terbutaline and Fenoterol reversed the ACh-induced bronchoconstriction only after the α-receptors had been blocked by phentolamine (10-5-10-4g/ml).