The airway response and cardiovascular and hypokalaemic effects of fenoterol, salbutamol, and terbutaline given in multiples of standard doses from metered-dose inhalers were studied in ten patients with mild asthma. In a double-blind, crossover, placebo-controlled study the subjects received 2, 6, and 18 puffs of each drug with intervals of 90 min, and forced expiratory volume in 1 s, heart rate, QTc interval, plasma potassium concentration, tremor, and bronchial reactivity to histamine were measured. All three drugs produced similar bronchodilatation. However, the rises in heart rate, QTc interval, and tremor and the fall in plasma potassium were greater after fenoterol than after salbutamol or terbutaline. The maximum mean (SD) increases in heart rate for fenoterol, salbutamol, and terbutaline were 29 (24) bpm, 8 (9) bpm, and 8 (14) bpm, respectively; falls in plasma potassium were 0.76 (0.62) mmol/l, 0.46 (0.32) mmol/l, and 0.52 (0.39) mmol/l, respectively. Fenoterol afforded no additional protection against histamine compared with salbutamol. These findings suggest that at doses based on those used in clinical practice fenoterol causes more adverse effects than salbutamol or terbutaline. The most likely explanation for these effects is that fenoterol has been marketed at a higher dose than the other beta 2-agonists; fenoterol may in addition be less selective for beta 2 receptors.