In a double-blind cross-over study comparing the duration of action of 12 micrograms and 24 micrograms formoterol, 500 micrograms terbutaline and placebo, lung function tests were performed in 12 never-smokers with non-allergic bronchial asthma. All the patients were hyperreactive to methacholine, had normal serum IgE level, and negative skin or RAST results. The lung function tests were carried out over an 11-h period and included measurements of lung volumes, airway resistance, dynamic spirometry, nitrogen single breath wash-out test and single breath diffusion capacity for CO. The airway resistance and maximal forced expiratory flow tests indicated a pronounced bronchodilator effect without adverse effects of 12 micrograms and 24 micrograms formoterol on both large and peripheral airways. Improved intrapulmonary gas distribution measured by the nitrogen wash-out test and more even distribution of the lung volumes indicating more homogeneous ventilation was seen for approx. 3 h after terbutaline and approx. 11 h after 24 micrograms formoterol. Measurement of the CO diffusion capacity indicates more effective gas diffusion and a better ventilation-perfusion ratio for at least 11 h, especially after inhalation of 24 micrograms formoterol.