Increased shear stresses are a hallmark of flow conditions in blood recirculating devices, and patients implanted with such devices require lifelong anticoagulation to counteract the high risk of thromboembolism. Despite this limitation, manufacturers only test for hemolysis in order to optimize device design. This is primarily due to the paucity of data regarding platelet damage, a major precursor of thromboembolism. Several studies have shown that platelets exposed to constant shear stress for a specified duration become activated. However, post high-shear stress activation behavior of platelets, which is critical to understanding increased thrombotic risk associated with blood recirculating devices, is poorly understood.Purified platelets were briefly exposed to high shear stress followed by longer periods of low shear stress. Platelet activity state (PAS) was measured using a modified prothrombinase assay. Platelets subject to an initial shear stress of 60 dyne/cm<super>2</super> and higher for 40 s showed minimal activation, but they exhibited a post-exposure activation rate that was significantly higher than for platelets that had not suffered the initial high shear insult. This sensitization response, although not significant, was also observed for platelets exposed to 1,000 dyne/cm<super>2</super> for 25 ms, similar to conditions found in blood recirculating devices. Shear-induced platelet sensitization was amplified when platelets were pre-treated with biochemical agonists thrombin receptor activating peptide (TRAP) and collagen, and attenuated with the addition of apyrase. This indicates physiological modulation of platelet activation after pathological shear stress exposure.A model predicting platelet activation in response to shear stress, duration of exposure, and shear-loading rate was derived and optimized using in vitro sensitization results. These observations and predictive model may provide blood recirculating device manufacturers with a tool to optimize product design for minimizing platelet response and thromboembolic complications rather than hemolysis.