Aim To observe the activity and expression of platelets nitric oxide synthase (NOS) in patients with acute coronary syndrome (ACS). Methods Peripheral venous blood was collected from both normals and ACS patients. Sepharose CL-2B gel colomn were used to isolate platelets. Nitric oxide synthase (NOS) activity in platelets was assessed by measuring the conversion of 3 H-L-arginine to 3 H-L-citrulline and determined in the absence or presence of histamine or L-NAME; The levels of endothelial NOS (eNOS) expression were measured by western blotting. Results Platelet NOS activity was significantly decreased in ACS patients compared with normal subjects (p<0.01); it was still lower after histamine stimulation (p<0.01); There were no significant change of eNOS expression between ACS patients and normal subjects. Conclusions Platelets NOS activity of ACS patients reduced. Since platelet aggregation and thrombus formation are implicated in unstable angina and myocardial infarction, impaired platelet NOS activity and thus decreased NO production may contribute to development of ACS.