There is a lot of controversy about potassium cardioplegic solution in its components and its method of perfusion. Although its effect emphasized in many reports, the optimal potassium level is still in question. In order to determine the optimal level of potassium, the isolated rat hearts were preserved in relatively bad conditions; such as 37 degrees C of infusates, non-oxygenation and continuous perfusion. The hypothermic effect on its optimal level was also studied by just reducing the temperature of the infusates to 4 degrees C. Thirty isolated rat hearts were divided into six groups of 5 hearts each. Each group received a different concentration (15, 25, 40, 60, 80, 100 mEq/L) in the cardioplegic solution. Using an isolated rat heart preparation, hemodynamic indices after 30 minutes preservation were compared with the previous control values of the same hearts. And the same protocol was performed on the profound hypothermia group using another 30 rat hearts. When Basic-Modified-Krebs-Solution was used as a base for the cardioplegic solutions, we concluded from the results that the solution' with 40 mEq/L of potassium chloride might offer the best myocardial protection of all concentrations tested; and that the safe range of the potassium was from 25 mEq/L to 60 mEq/l in the profound hypothermic preservation.