This double-blind, randomised, within patient, placebo-controlled study set out to investigate the effect of a cardioselective beta-blocker, atenolol, at different oral doses (50, 100 and 200 mg) and a non-selective agent, propranolol (40 mg), upon 1. airways resistance (forced expiratory volume at one second = FEV1) and 2. the bronchodilator action of increasing doses of inhaled isoprenaline, in patients with co-existent hypertension and reversible airways obstruction. In 10 patients, two hours after drug administration, the 3 doses of atenolol caused a significantly greater (P less than 0.05) degree of B1-blockade than propranolol. In contrast the 3 doses of atenolol caused significantly less (P less than 0.05 to 0.01) B2-blockade as evidenced by a smaller fall in FEV1. The isoprenaline FEV1 dose response curves were displaced progressively to the right of the placebo curve with increasing doses of atenolol, but the greatest displacement was with propranolol. It was concluded that patients with reversible airways obstruction who require beta-blockade should be given a low dose of a cardioselective agent in conjunction with, if required, a beta 2 stimulant such as isoprenaline. Such a treatment will be less likely to cause a troublesome increase in airways resistance and the bronchodilator action of the beta 2 stimulant will be almost fully preserved.