Abstract:The degree of myocardial cell necroses in pigs exposed to stress induced by a myorelaxant (succinylcholine) for about 12 min. was investigated. The β‐adrenoceptor blocker propranolol was given in varying doses prior to the induced stress. The degree of necroses in these pigs was evaluated according to a point scale, ranging from 0 to 6 points, and compared with the changes found in untreated controls following stress. The blood levels of noradrenaline and adrenaline were assayed before, during, and immediately after the period of stress. No protective effect of propranolol on the cardiac lesions was found when the drug was administered intravenously in a single dose of either 1 mg or 3 mg per kg. During the period of stress the blood levels of noradrenaline were high, indicating a high degree of activity in the sympatho‐adrenal system. Propranolol in a single dose of 10 mg per kg intravenously reduced the number and size of heart lesions, while the blood levels of the catecholamines were lower under stress than after administration of 1 and 3 mg propranolol. After oral treatment with propranolol, 120 mg three times a day for 6 or 7 days, an almost complete absence of heart cell necrosis after the stress was obtained. The activity of the sympatho‐adrenal system was elevated, as reflected by the high blood levels of catecholamines. The results indicate that stress‐induced heart cell necrosis is mediated via the activity of the sympatho‐adrenal system. The difference between a single and repeated doses of propranolol in protecting the heart suggests a complex ation of the adrenergic β‐rec