30 open-chest rabbits were randomly divided into 3 groups. In group Ⅰ the left anterior descending coronary artery (LAD) was false ligated. Rabbits of group Ⅱ and Ⅲ underwent 30 minuts of LAD occlusion and 4h of reperfusion. After LAD ligated in group Ⅲ were intravenously administered nitroglycerin and in group Ⅰ and Ⅱ were sodium chloride. The level of plasme nitric oxide (NO), alpha-granule membrance protein (GMP-140) on the surface of platelet and thromboxane B_2 (TXB_2) were determined in some time points before and after operation. The results showed that the level of NO, GMP-140 and TXB_2 in group Ⅰ were not difference before and after operation. In group Ⅱ and Ⅲ the level of NO was significantly decreased and the level of the GMP-140 and TXB_2 were significantly increased after myocardial ischemia and reperfusion. Changes in group Ⅱ were more marked than that in group Ⅲ. There were close relation between the value of NO and the value of GMP- 140 and TXB_2. It suggested that NO inhibited platelet activation after myocardial ischemia-reperfusion in vivo.