The work described in this Thesis was initially motivated by the observation of the too often occurrence of systemic hyperkalemia at the end of cardiac operations which employed cardiopulmonary bypass and hyperkalemic cardioplegic arrest. These observations were gathered mainly during five years of training as a Cardiovascular Surgeon, however I have witnessed its appearance in other Centres I had the opportunity to visit. Esmolol, (Brevibloc(R), Zeneca Pharma Canada, Inc.) was the alternative drug chosen to be tested simply because it is the only drug available commercially that possesses the required effects and has a ultra-short acting period. Esmolol, in commercial presentation vials, was the only gift from Zeneca Pharma Canada Inc., and this study was not the result of a contractual research agreement. The following pages will describe, after a dissertative background on aspects of Cardiac Physiology pertinent to the work pursued, the results of three consecutive studies designed to investigate whether Esmolol has cardioplegic property, if this effect is toxic and what mechanism(s) may underlie its action.