Platelets, on activation by endothelial damage, release ADP, ATP, serotonin, epinephrine, and norepinephrine. Although ATP is known to augment the action of norepinephrine in cardiovascular and endocrine systems, the possible interaction between ATP and catecholamines in regulation of platelet reactivity has not been reported. The addition of ATP (1-5 µM) to human platelet-rich plasma did not induce platelet aggregation; however, it 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 adenosine 3'-phosphate 5'-phosphosulfate, a selective P₂Y₁ receptor antagonist. ATP alone did not cause release of intracellular Ca²⁺, but produced a significant Ca²⁺ response in the presence of norepinephrine. In contrast, the P₂X₁ receptor agonists P¹,P⁶-diadenosine-5' hexophosphate and ,-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.