Beta-adrenergic blocking agents are widely used to treat disorders of cardiac rhythm and rate, angina, and hypertension. Propranolol is the most widely used beta-adrenergic blocking agent in this country. Because of its nonselective beta-adrenergic blocking effect, propranolol may be associated with significant bronchoconstriction in asthmatic subjects and in some patients with chronic obstructive pulmonary disease. Since tolamolol, a new beta-adrenergic blocking agent, has cardioselectivity in animals, we studied asthmatic subjects for six hours on three separate days in a double-blind crossover comparison of oral therapy with 40 mg of propranolol, its beta-adrenergic blocking equivalent dose of tolamolol (50 mg), and a high dose of tolamolol (100 mg). All three dosages had equipotent effects on heart rate and systolic pressure. The 50-mg dose of tolamolol had no effect on pulmonary function over six hours; however, both propranolol (40 mg) and the 100-mg dose of tolamolol had equivalent deleterious effects on airway resistance and on rates of expiratory flow. We conclude that the cardioselectivity of tolamolol is dose-limited but is present at the dosage of 50 mg, which is equivalent to the usual antiarrhythmic beta-adrenergic blocking dose of propranolol (40 mg).