Using heparinized whole blood and flow conditions, it was shown that adenosine 5′-diphosphate (ADP) receptors P2Y₁₂ and P2Y₁ are both important in direct shear-induced platelet aggregation and platelet aggregation subsequent to initial adhesion onto von Willebrand factor (vWf)–collagen. In the viscometer, whole blood was subjected to shear rates of 750, 1500, and 3000 s⁻¹ for 30 seconds at room temperature. The extent of aggregation was determined by flow cytometry. The P2Y₁₂antagonist AR-C69 931MX (ARMX) reduced shear-induced aggregation at these rates by 56%, 54%, and 16%, respectively, compared to control samples. Adenosine 3′,5′-diphosphate (A3P5P; P2Y₁antagonist) inhibited shear-induced aggregation by 40%, 30% and 29%, respectively, compared to control samples. Blockade of both ADP receptors at 3000 s⁻¹ with ARMX plus A3P5P further reduced the platelet aggregation by 41% compared to the addition of ARMX alone (57% compared to control samples). Using a parallel-plate flow chamber, whole blood was perfused over bovine collagen type 1 at a wall shear rate of 3000 s⁻¹ for 60 seconds. Platelet deposition was quantified with epifluorescence video microscopy and digital image processing. Blockade of P2Y₁₂ alone or blockade of P2Y₁ alone did not reduce thrombus formation on vWf–collagen. In contrast, blockade of both P2Y₁₂ and P2Y₁ reduced platelet deposition by 72%. These results indicate that combinations of antagonists of the ADP receptors P2Y₁₂ and P2Y₁ are effective inhibitors of direct shear-induced platelet aggregation and of platelet aggregation subsequent to initial adhesion under flow conditions. Inhibitors of these pathways are potentially useful as antiarterial thrombotic agents.