Platelets, upon activation by endothelial damage, release ADP, ATP, serotonin, epinephrine, and norepinephrine. Although ATP is known to augment action of norepinephrine in cardiovascular and endocrine systems, possible interaction between ATP and catecholamines in regulation of platelet reactivity has not been reported. Addition of ATP (1-5 µM) to human platelet-rich plasma did not induce platelet aggregation, but selectively augmented the aggregatory response to norepinephrine and epinephrine, but not to serotonin. This potentiating action of ATP was dose-dependent and was not due to contamination by, or hydrolysis to, ADP. The action of ATP was blocked by 10 µM of A3P5PS, a selective P₂Y₁ receptor antagonist. ATP alone did not cause release of intracellular calcium, but produced a significant calcium response in the presence of NE. In contrast, the P₂X₁ receptor agonists, Ap6A and ,ß-methylene-ATP had no effect on norepinephrine-induced platelet aggregation even when added at 100 µM. This synergistic interaction between ATP and norepinephrine in stimulating platelet aggregation may have significant clinical implications and suggests a prothrombotic role for ATP in stress.