The common denominator concerning the changes of myocardial function during anaesthesia is the activity of the sympatho-adrenal system. Evidently, all anaesthetic agents at equipotent dose levels will depress myocardial function in the isolated heart, showing their direct effect upon the myocardium. However, the depressant effects of anaesthetic drugs in man are antagonized by the responses of cardio-regulatory mechanisms. Myocardial depression in man becomes evident when the compensatory mechanisms are blocked.