Abstract:Rats received continuous infusions of β‐adrenoceptor agonists by means of Alzet osmotic mini‐pumps implanted subcutaneously. After 7 days of isoprenaline infusion (2 mg/kg per day) the heart/body weight ratio increased about 40 per cent compared with placebo treatment. The difference persisted after freeze‐drying indicating a true hypertrophy and not merely an oedema. When terbutaline (20 mg/kg per day) was substituted for isoprenaline, the increase in wet weight ratio reached only about 5 per cent. Procaterol (2 mg/kg per day) and pirbuterol (20 mg/kg per day) had no effect on the heart weight. It is concluded that in doses expected to cause comparable stimulation of β2‐adrenoceptors the unselective agonist isoprenaline is able, more than the β2‐selective agonists terbutaline, procaterol and pirbuterol to cause cardiac hypertrophy thus indicating the involvement of β1