Thirteen patients given long-term propranolol hydrochloride therapy for heart disease required 15 abdominal surgical procedures. On each occasion, propranolol therapy was maintained postoperatively by continuous intravennous infusion. Duration of infusion ranged from one to nine days, and each infusion was monitored with frequent measurements of serum propranolol concentrations. In patients with normal hepatic and renal function, therapeutic serum propranolol levels were attained with a narrow dose range averaging 3.0 mg/hr, irrespective of body weight. All patients had postoperative courses free of complications attributable to β-blockade. This form of therapy appears to protect against sympathetic stimulation during the perioperative period and to prevent the propranolol withdrawal syndrome in such patients. Continuous propranolol infusion might also be useful in other clinical situations, such as acute aortic dissection or severe thyrotoxicosis, where predictable therapeutic serum propranolol levels could be maintained when oral therapy was contraindicated.