Abstract
1. The haemodynamic effects of propranolol, practolol and 4-hydroxypropranolol have been compared in anaesthetized dogs both at free heart rates and at heart rates fixed by electrical pacing. All three drugs produced a fall in heart rate at the lowest dose of 0.09 mg/kg.2. Practolol caused less change in dP/dt, cardiac output and blood pressure than did propranolol. 4-Hydroxypropranolol had intermediate effects. Since the changes in dP/dt caused by these three drugs were still present when the heart rate was fixed by pacing, it is concluded that these were independent of changes in heart rate.3. The fact that practolol and 4-hydroxypropranolol cause less change in haemodynamic function than propranolol may be associated with the intrinsic sympathomimetic activity possessed by the first two of these drugs.