Twenty eight patients were randomly allocated into treatment or placebo groups. The treatment group received a single 400 mg oral dose of acebutolol, the placebo group an identical inert prepation. A blood sample from each patient was subsequently analysed for plasma concentrations of acebutolol and diacetolol, and the electrocardiogram was continuously monitored for changes of rate and rhythm. The plasma concentrations of acebutolol and diacetolol were all above the previously reported minimum levels for effective dysrhythmia control and the nine patients in whom irregularities of rhythm occurred were all in the placebo group. One patient in the treatment group, who was 58 years old, required intravenous injection of atropine for bradycardia and hypotension after induction of anaesthesia. It is suggested that in patients with small body weights and in those patients over 50 years of age the predmedication dose of acebutolol should be reduced to 200 mg.