The effect of glutamic acid added to cardioplegic solution containing 20 mM K+ on the cardiac function and metabolism was studied in isolated working rat hearts. 30-min cardiac arrest resulted in profound fall in creatine phosphate and ATP content, by four- and two-fold, respectively, as well as in four-fold rise in AMP content. Simultaneously, during cardioplegia a decline in tissue glutamate and aspartate content and an increase in tissue ammonia and alanine content were found. After reperfusion, an incomplete restoration of ATP, AMP, and creatine phosphate content were observed; the cardiac output recovered only to 39 percent of the initial value. An addition of glutamic acid to cardioplegic solution was associated with significantly less decline in the content of high-energy phosphates and less prominent rise in AMP content during cardioplegia. It also prevented the decline in tissue aspartate content and caused a lesser ammonia accumulation in myocardial tissue due to the activation of glutamine synthesis. In spite of this the tissue ammonia level remained elevated. Reperfusion with Krebs-Henseleit buffer resulted in the recovery of cardiac output to 75% of the initial value as well as better restoration of high-energy phosphate content. The addition of glutamic acid in the perfusate during reperfusion led to further improvement of ATP and creatine phosphate content. It is suggested that an addition of glutamic acid may have beneficial effect in open heart surgery.