The effects of newly discovered vasoconstrictor peptide endothelin was studied on human, rabbit and canine platelet function. Endothelin (0.01 nM-1 microM) did not promote platelet aggregation. In human platelets, endothelin (0.1 microM) did not significantly affect aggregation responses to ADP, collagen, epinephrine, arachidonic acid, PGH2 or thrombin. Endothelin did not promote the mobilization of intracellular calcium in Fura2 loaded human platelets. In rabbit and canine platelets endothelin produced significant potentiation of platelet aggregation mediated by low concentrations of ADP. Aggregation responses to higher concentration of ADP (5 microM) were unaffected by endothelin. These data reveal that under certain circumstances endothelin may potentiate rabbit and canine platelet aggregation responses to ADP, however endothelin does not produce direct effects on human platelet function.