The relation between airway responsiveness to propranolol and histamine was studied in 32 asthmatic children. Propranolol and histamine were given by nebuliser to a maximum dose of 16 mg/ml and 32 mg/ml respectively and the response was measured as the provocative concentration of agonist causing a 20% fall in FEV1 (PC20). A PC20 histamine value of less than 32 mg/ml was obtained in 24 of the 32 children, of whom 15 had a measurable PC20 propranolol (less than 16 mg/ml). In these 24 children the geometric mean PC20 histamine was 4.5 mg/ml and 14.4 mg/ml respectively in those with and without a measurable PC20 propranolol (p = 0.023). There was a linear relationship between histamine and propranolol PC20 values (r = 0.60), and between PC20 histamine and FEV1 % predicted (r = 0.43), but not between PC20 propranolol and FEV1 % predicted (r = 0.38). In an open time course study in 12 children with asthma recovery of FEV1 after inhaled propranolol was incomplete in seven of the children after 90 minutes. When inhaled propranolol was followed by inhaled ipratropium bromide in a further 11 children FEV1 had returned to baseline in all children after 60 minutes. Thus propranolol inhalation can be used in children with asthma to assess the contribution of the beta adrenergic system to the regulation of bronchial smooth muscle tone. The test has several disadvantages in comparison with histamine provocation-long duration, the prolonged action of propranolol, and the fact that only the children with substantial hyperreactivity to histamine react to propranolol.