1 The beta-adrenoceptor blocking agent, acebutolol (M & B 17,803), has been compared with propranolol, practolol, lignocaine and quinidine for its ability to revert or prevent various types of experimental arrhythmias.2 By intravenous infusion, acebutolol had one half the potency of propranolol in reverting an established ouabain-induced ventricular arrhythmia in the anaesthetized dog. Practolol was ineffective in the conditions used.3 High oral doses of acebutolol or propranolol significantly increased the arrhythmic dose of ouabain in the conscious rabbit. Similar doses of practolol produced a significant decrease (i.e. potentiation) in the dose of ouabain required to produce arrhythmia. Lignocaine and quinidine showed no or little activity in this test.4 Propranolol, acebutolol and practolol were all effective in decreasing the frequency of ectopic beats induced by adrenaline and methylchloroform in the anaesthetized cat. Lignocaine and quinidine were only weakly effective.5 Acebutolol and propranolol were equally effective either intravenously or orally in reducing the incidence of ventricular fibrillation produced by chloroform in mice.6 It is suggested that the wide spectrum of experimental anti-arrhythmic activity of acebutolol coupled with its cardioselectivity may make it an interesting compound in the treatment of cardiac arrhythmias in man.