OBJECTIVE: To analyse if platelet responsiveness to aspirin (ASA) may be associated with a different ability of platelets to generate nitric oxide (NO). PATIENTS/METHODS: Platelets were obtained from 50 patients with stable coronary ischemia and were divided into ASA-sensitive (n = 26) and ASA-resistant (n = 24) using a platelet functionality test (PFA-100). RESULTS: ASA-sensitive platelets tended to release more NO (determined as nitrite + nitrate) than ASA-resistant platelets but it did not reach statistical significance. Protein expression of nitric oxide synthase 3 (NOS3) was higher in ASA-sensitive than in ASA-resistant platelets but there were no differences in the platelet expression of nitric oxide synthase 2 (NOS2) isoform. The highest NOS3 expression in ASA-sensitive platelets was independent of the presence of T-to-C mutation at nucleotide position -786 (T(-786) → C) in the NOS3-coding gene. However, platelet content of phosphorylated NOS3 at Serine (Ser)(1177), an active form of NOS3, was higher in ASA-sensitive than in ASA-resistant platelets. The level of platelet NOS3 Ser(1177) phosphorylation was positively associated with the closure time in the PFA-100 test. In vitro, collagen failed to stimulate the aggregation of ASA-sensitive platelets, determined by lumiaggregometry, and it was associated with a significant increase (p = 0.018) of NOS3 phosphorylation at Ser(1177). On the contrary, collagen stimulated the aggregation of ASA-resistant platelets but did not significantly modify the platelet content of phosphorylated NOS3 Ser(1177). During collagen stimulation the release of NO from ASA-sensitive platelets was significantly enhanced but it was not modified in ASA-resistant platelets. CONCLUSIONS: Functional platelet responsiveness to ASA was associated with the platelet content of phosphorylated NOS3 at Ser(1177).