Increased hypoxia has been found after beta 2 adrenoceptor agonists (but not adrenaline) in asthmatics. Combined with hypokalaemia and sympathomimetic stimulation, this may predispose to cardiac arrhythmias. We have compared the effects of nebulized adrenaline and a selective beta 2 agonist (salbutamol) on the arterial oxygen saturation (SaO2), minute ventilation (VE), forced expiratory volume in 1 s (FEV1), plasma potassium and the electrocardiogram (ECG) in patients with chronic stable asthma. Six patients were studied according to a randomized, placebo-controlled, double-blind cross-over protocol. Adrenaline (5 mg), salbutamol (5 mg) and placebo were administered during 4 min tidal breathing using a nebulizer driven by air. There was a fall in SaO2 after both adrenaline (mean % fall (SEM) 3.3 (0.2)) and salbutamol (4.0 (0.7)) associated with an increase in FEV1, with no change in VE. Therefore, the fall in SaO2 must have been caused by increased ventilation-perfusion imbalance. There was an increased heart rate after both adrenaline and salbutamol and ventricular ectopic beats and a short run of parasystole were recorded on the ECG in one patient after adrenaline and in two patients after salbutamol. No change was found in plasma potassium levels. We conclude that both adrenaline and a selective beta 2 agonist salbutamol can cause a fall in SaO2 and ventricular ectopy in some asthmatic patients.