SUMMARY1. The effects of pre‐ and post‐treatment with naloxone on the cardiotoxicity of ouabain in the guinea‐pig were studied.2. After pretreatment with naloxone, the dose of ouabain required to induce ventricular arrhythmias and cardiac arrest were significantly increased, in a dose‐dependent manner, compared with the control, indicating a protective effect of naloxone against digitalis intoxication.3. Administration of naloxone at the onset of cardiac arrhythmias induced by a lethal dose of ouabain restored the cardiac rhythm and consequently saved life in seven out of eight animals, indicating an antiarrhythmic effect of naloxone in digitalis‐intoxicated guinea‐pigs.4. The protective and antiarrhythmic effects of naloxone against digitalis intoxication have clinical i