Hydrolysis of acetylsalicylic acid (ASA, aspirin), an antiplatelet drug commonly used in the prevention of stroke and myocardial infarction, seems to play a crucial role in its pharmacological action. Thirty-eight healthy volunteers and 38 type 2 diabetic patients were enrolled to test the hypothesis that the enhanced plasma degradation and lowered bioavailability of ASA in diabetic patients is associated with the attenuation of platelet response. Aspirin esterase activities were tested at pH 7.4 and 5.5. A significantly higher overall aspirin esterase activity was noted at pH 7.4 in the diabetic patients (P<0.003), corresponding to faster ASA hydrolysis (P<0.006). This increased activity was attributable to butyrylcholinesterase and probably to albumin, because it was effectively inhibited by eserine and 4-bis-nitrophenyl phosphate (P<0.01). No significant differences between control and diabetic subjects were found at pH 5.5 in either enzymatic activities or ASA hydrolysis rates. The enhanced plasma ASA degradation in diabetic subjects was significantly associated with the refractoriness of blood platelets to ASA (P<0.05) and modulated by plasma cholesterol (P<0.01). No direct effects of plasma pH or albumin were observed. In conclusion, higher aspirin esterase activity contributes to the lowered response of diabetic platelets to ASA-mediated antiplatelet therapy.