1. In this double-blind randomized study, after a 4-weeks placebo period, 18 patients with mild to moderate primary hypertension were assigned to treatment with either dilevalol (n = 9) daily or atenolol (n = 9) over a period of 3 months. 2. Expiratory flows, lung volumes and airway responsiveness (AR) to methacholine were assessed at the end of the placebo period and after an active treatment of 12 weeks. Blood pressure (BP), heart rate (HR) and ECG were monitored during the methacholine challenges. Twice daily peak expiratory flow rates and respiratory symptoms were recorded on a diary card. 3. No significant effects on ECG, HR and BP were observed after methacholine inhalation. In all but one subjects there was no significant change in expiratory flows, lung volumes or AR throughout the study. Mean FEV1, FVC, PEFR, FRC and PC20 methacholine were unchanged after 3 months of treatment, and not statistically different between patients on dilevalol or atenolol. 4. One subject, without previous history of asthma, developed transient airflow obstruction 8 weeks after beginning dilevalol. 5. Dilevalol and atenolol have no significant effects on pulmonary function and AR in most subjects with no baseline airflow limitation. 6. Airflow obstruction may develop in normal subjects on dilevalol. Methacholine challenges are safe in subjects with uncomplicated hypertension.