Patients undergoing hypothermic cardiopulmonary bypass are often receiving aspirin therapy. Hypothermia, aspirin and cardiopulmonary bypass can each induce a platelet function defect, but it is not known if the effects of aspirin and hypothermia are additive in this regard. To address this question in humans in vivo, the forearm skin temperature of healthy volunteers was equilibrated and maintained at either normothermia (32 degrees C) or hypothermia (28 degrees C or 22 degrees C) before and 16 h after the ingestion of 650 mg aspirin. A standardized template bleeding time was performed on the forearm and the shed blood emerging from the wound was assayed for platelet surface P-selectin expression by whole blood flow cytometry (reflecting alpha granule secretion) and thromboxane B2 (the stable metabolite of thromboxane A2) by radioimmunoassay. Hypothermia resulted in marked prolongation of the bleeding time. Aspirin resulted in prolongation of the bleeding time under normothermic conditions, but only minimally augmented the hypothermia-induced prolongation of the bleeding time. Platelet surface P-selectin up-regulation in shed blood was abolished by hypothermia. Aspirin had no effect on maximal platelet surface P-selectin expression under normothermic or hypothermic conditions. Both hypothermia and aspirin resulted in markedly reduced shed blood thromboxane B2. Although aspirin slightly augmented the hypothermia-induced reduction in shed blood thromboxane B2, the concentration of thromboxane generated in shed blood under hypothermic conditions in the absence of aspirin had no effect on platelet surface P-selectin or platelet aggregation in whole blood. In conclusion, as determined by three independent parameters of the shed blood emerging from a standardized bleeding time wound (bleeding time, platelet surface P-selectin, and thromboxane B2), aspirin did not significantly augment hypothermia-induced platelet dysfunction in vivo.