Magnesium is an essential intracellular cation playing an important role in most cellular energy transfer reactions and in maintaining membrane potentials. To determine the effects of CPB on serum Mg levels with other electrolytes and the effects of CPB on serum Mg levels with other electrolytes and the effects of supplementing the priming solution with 1.5gr of MgSO4 we planned a two stage study. In the first stage Mg, Ca, K, and Na levels were compared in two separate groups of 15 patients with one receiving the Mg supplement and the other not. The results showed that in both groups serum Mg levels fell during and after CPB but the fall in the Mg supplement group was less and the difference was statistcally significant. In the second stage of the study the arrhythmia analysis during the postoperative first 6 hours of two groups of coronary bypass patients, one with Mg supplementation and the other without, were compared and the group with magnesium supplementation and the other without, were compared and the group with magnesium suplementation had less atrial and ventricular arrhythmia with less antiarrhyhthmic drug use compared to the non supplement group. As a result of this study we believe simple Mg suplementation with 1.5 gr. of MgSO4 in the priming solution results in a rise in the serum Mg levels during and after CPB and that this method of suplementation is effective in reducing the incidence of arrhythmia in the early postoperative period.