Using the ³¹P-topical nuclear magnetic resonance method (TMR), an attempt was made to determine the myocardial high-energy phosphate compounds (HEP) contents under in situ conditions in closed-chest animals, and the effects of opening the thorax on the myocardial energy metabolism were studied comparing the cardiotoxic effects of 5-fluorouracil (5-FU) in closed-chest and open-chest animals. It was found that the depletion of myocardial HEP produced by 5-FU was much more marked in open-chest animals than in closed-chest ones, indicating the necessity of conducting the experiments in closed-chest animals for the proper evaluation of the cardiotoxicity of certain types of compounds. Therefore, the cardiotoxicity of a prodrug of 5-FU was assessed in closed-chest animals, and it was found to be less cardiotoxic than 5-FU.