Red blood cells (RBC) contribute significantly to hemostasis and thrombosis under oscillatory flow conditions, and erythrocytosis has been associated with increased thrombotic risk. To capture dynamic effects of RBC on platelets, we used a recently described Cone and Plate (let) Analyzer (CPA), to evaluate the effect of hematocrit (Hct) on platelet function in whole blood under arterial flow conditions (1800 sec-1, 2 min, 25°C). Anticoagulated blood, reconstituted to varying hematocrits with autologous RBC, demonstrated a significant increase (30–50%, n= 12, p < 0.05) in adherent platelet aggregate size at Hct levels >45%. No significant effect on platelet adhesion was noted. Increases in aggregate size were not accompanied by significant platelet dense or alpha granule secretion, measured using 14C-serotonin release and Platelet Factor 4 ELISA assays, respectively. However, when cell-free supernatants (SNT) were prepared post shearing to investigate the presence of proaggregatory activity by adding SNT to fresh PRP (1:10 SNT: PRP ratio), and quantifying the formation of small platelet aggregates with a laser light scatter aggregometer, SNT derived from sheared high Hct samples contained nearly twice (1.75 + 0.16, n=5, p < 0.05) the activity of sheared normal Hct samples. The erythrocytosis-induced increase in platelet aggregate size was not affected by pretreatment of blood with 0.05 mM aspirin, but could be prevented completely by antagonism of P2Y1 (MRS2179, 200 μM), P2Y12 (2-methylthioadenosine 5′-monophosphate, 10–100 μM), or P2X1 (MRS2159, 200 μM), ADP and ATP receptors, respectively, as well as by converting exogenous ADP to ATP with a combination of creatine phosphate (2 mM) and creatine phosphokinase (20 U/ml). Whereas negligible platelet granule secretion was measured post shear, metabolic inhibition of RBC with 1% sodium azide or 0.25% glutaraldehyde fixation (2 h, 37°C) fully inhibited erythrocytosis-enhanced increases in platelet aggregate size. Thus, the adenine nucleotides contributing to erythrocytosis-enhanced platelet aggregate formation under physiologic shear appear to be derived predominantly from RBC. Taken together, the data suggest that ADP and ATP are both required for erythrocytosis-induced large platelet aggregate formation, but that the presence of ATP alone is not sufficient. The results may further suggest a role for direct ADP or ATP receptor blockade for the prevention of erythrocytosis-associated thrombotic complications.