The effect of metoprolol on pulmonary function was compared with that of propranolol and placebo in twelve patients with chronic obstructive lung disease. Propranolol caused a significant reduction in FEV1 compared with metoprolol and placebo. Metoprolol gave a slight reduction compared with placebo. FVC was not significantly influenced by metoprolol, but propranolol caused a significant depression compared both with metoprolol and placebo. The isoprenaline-mediated increase in FEV1 and FVC was not significantly altered after metoprolol compared with placebo. After propranolol the isoprenaline-mediated increases in FEV1 and FVC were 40 and 24% lower respectively than after metoprolol and placebo. Unwanted effects and pulmonary symptoms were at an acceptable level in all cases except in two patients who had to be given i.v. theophyllamine therapy during propranolol treatment. It is concluded that metoprolol exhibits a selective beta1-blocking action and thus may be given to patients with chronic obstructive lung disease provided that adequate beta2-stimulator treatment is given concomitantly.