1. Patients were grouped into categories of 'no airways disease', 'obstructive airways disease without response to bronchodilator' and 'obstructive airways disease with bronchodilator responsiveness'. 2. Cyclic nucleotides were assayed in specimens of lung tissue that were excised during surgery. 3. Reduced levels of adenosine 3',5'-cyclic monophosphate (cyclic AMP) were found in pulmonary tissue obtained from patients with reversible obstructive airways disease, lending support to the beta-adrenergic theory of asthma.