The mechanism of myocardial protection provided by red blood cells was studied with a preparation of isolated, blood-perfused dog heart. Myocardial function, metabolism, and high-energy phosphate stores were determined before and at the end of cardioplegic arrest and after reperfusion. Cardioplegic solutions containing NaCl, KCl, and dextran (crystalloid; C) were compared, at 27 degrees C, with an identical solution containing 20% washed red cells (R) and a third solution in which the red cells were treated with 0.02M NaNO2, resulting in methemoglobin (M) formation or red cells able to buffer pH but unable to transport oxygen. Solutions were delivered at 15 min intervals during 2 hr of arrest and coronary effluent pH, PCO2, and lactate were measured. Both R and M solutions prevented the increases in effluent PCO2 and decreases observed in pH with C. On the other hand, mechanical, metabolic, and high-energy phosphate determinations revealed that marked protection was provided by R but not by M or C. Thus, myocardial protection provided by R at 27 degrees C is not the result of buffering but appears to be primarily the result of oxygen delivery by the red cells to the myocardium.