Phenylephrine, a powerful alpha receptor stimulant, has been shown to cause a significant fall in the FEV1 and SGaw in six patients with extrinsic bronchial asthma after prior beta blockade with propranolol. In contrast, propranolol or phenylephrine after prior beta blockade failed to effect a significant change in the FEV1 and SGaw in five normal subjects. The phenylephrine effect can be completely inhibited by alpha receptor blocking drugs, phenoxybenzamine and thymoxamine. These observations suggest that the bronchomotor tone in asthma is largely controlled by the sympathetic activity and that there are alpha receptors in the human airways which in the presence of beta blockade can be stimulated to give bronchoconstriction.