Tetrahydroacridine (THA), or Cognex, is currently awaiting FDA approval for the treatment of Alzheimer's disease. In addition to reports indicating that THA improves the symptoms of patients with Alzheimer's disease, we have found that THA possesses potent antiplatelet activity. THA produced dose-dependent inhibition of human platelet aggregation induced by collagen, ADP, A23187, and phorbol ester. THA, when added to activated platelets, dispersed the platelet aggregates. We have also examined the effects of THA on intracellular Ca++ mobilization, ATP release, and production of cyclic AMP.