The role of beta-adrenoceptors in the mechanisms of bronchial hyperresponsiveness was studied in 20 normal subjects. Propranolol 120 mg b.d. or placebo was taken for one week. Four bronchial provocation tests: exercise, inhalation of nebulised water, histamine, and methacholine, were carried out before and during the last 2 days of the treatments. FEV1 and flow rate at 50% of vital capacity (V50) were measured. Cardio-vascular beta-blockade with propranolol, measured by reduction in exercise heart rate, was shown in 17 subjects. Propranolol enhanced bronchial responsiveness to some tests in some subjects, but there was no consistent pattern. For the group, the only significant increases in response were 1) the maximum fall in V50 on methacholine challenge (P less than 0.05, paired t-test, n = 17), 2) the dose of methacholine giving a 10% fall (PD10 value) in FEV1 (p less than 0.05 n = 11). These results do not support the concept of decreased beta-adrenoceptor responsiveness as a single lesion leading to abnormal bronchial reactivity.