Acting through cell surface receptors, ADP activates platelets resulting in shape change, aggregation, thromboxane A2 production, and release of granule contents. ADP also causes a number of intracellular events including inhibition of adenylyl cyclase, mobilization of calcium from intracellular stores, and rapid calcium influx in platelets. However, the receptors that transduce these events remain unidentified and their molecular mechanisms of action have not been elucidated. The receptor responsible for the actions of ADP on platelets has been designated the P2T receptor. In this study we have used ARL 66096, a potent antagonist of ADP-induced platelet aggregation, and a P2X ionotropic receptor agonist, alpha,beta-methylene adenosine 5'-triphosphate, to distinguish the ADP-induced intracellular events. ARL 66096 blocked ADP-induced inhibition of adenylyl cyclase, but did not affect ADP-mediated intracellular calcium increases or shape change. Both ADP and 2-methylthio-ADP caused a 3-fold increase in the level of inositol 1,4,5-trisphosphate over control levels which peaked in a similar fashion to the Ca2+ transient. The increase in inositol 1,3,4-trisphosphate was of similar magnitude to that of inositol 1,4,5-trisphosphate. alpha,beta-Methylene adenosine 5'-triphosphate did not cause an increase in either of the inositol trisphosphates. These results clearly demonstrate the presence of two distinct platelet ADP receptors in addition to the P2X receptor: one coupled to adenylyl cyclase and the other coupled to mobilization of calcium from intracellular stores through inositol trisphosphates.