1 Minute ventilation, alveolar PCO2, CO2 production and heart rate were measured in eight normal subjects before and during infusions of noradrenaline 0.2, 1.0 and 5.0 micrograms min-1 and isoprenaline 0.04, 0.2 and 1.0 microgram min-1. 2 These measurements were repeated after propranolol 3.5 mg, atenolol 8 mg or metoprolol 7 mg by intravenous injection. 3 Noradrenaline 5.0 micrograms min-1 and isoprenaline 1.0 microgram min-1 significantly increased ventilation and CO2 production and decreased alveolar PCO2. These changes were attenuated by propranolol, atenolol and metoprolol. There was no significant difference between the blocking effects of the three beta-adrenoceptor blockers for these three variables but propranolol was more effective than atenolol or metoprolol in blocking isoprenaline induced tachycardia (P less than 0.001). 4 The hyperventilatory response to catecholamines is predominantly a beta1-effect.