Objective: The aim of this study was to compare the effects of two different cardioplegic solutions on nitric oxide (NO) release from coro- nary vasculature in patients with type II diabetes mellitus undergoing coronary artery bypass grafting (CABG) surgery. Methods: Forty patients undergoing elective CABG surgery were randomized to be given crystalloid (Group 1 ) or blood (Group 2) cardiople- gia. Aortic and coronary sinus blood samples were taken at three different time periods and the release of NO from the coronary vascula- ture was determined by measuring its stable end-products, nitrite and nitrate. The difference between the aortic and coronary sinus con- centrations of nitrite and nitrate represents the amount of NO released by coronary vascular bed. Results: Before application of aortic cross-clamp, at T1 period, the levels of nitrite/nitrate from the coronary vasculature were similar in both groups (6.53±1.21 μM vs 6.07±1.24 μM, p> 0.05). However after the removal of cross-clamp, a significant decrease in NO was observed in Group 1 as compared with Group 2 (4.21±0.73 μM vs 4.92±1.02 μM, p< 0.01). This decrease persisted at T3 period, after 30 minutes of reper- fusion in group 1 being significantly different from group 2 (3.86±0.49 vs 4.37±0.72 μM, p<0.05). Conclusion: This study has shown that in patients with type II diabetes mellitus crystalloid cardioplegia causes a decrease in the release of NO from coronary vascular bed during aortic cross-clamp and reperfusion period whereas more physiologic blood cardioplegia did not. Our findingsindicatethatbloodcardioplegiaprotectsendothelialfunctionbetterthancrystalloidcardioplegiaindiabeticpatients.