Platelets from Gαq knockout mice are unable to aggregate in response to physiological agonists like adenosine 5′-diphosphate (ADP), thromboxane A₂, thrombin, or collagen, although shape change still occurs in response to all of these agonists except ADP. ADP-induced platelet aggregation results from simultaneous activation of the purinergic P2Y₁receptor coupled to calcium mobilization and shape change and of a distinct P2 receptor, P2cyc, coupled through Gi to adenylyl cyclase inhibition, which is responsible for completion and amplification of the response. P2cyc could be the molecular target of the antithrombotic drug clopidogrel and the adenosine triphosphate (ATP) analogs AR-C69931MX, AR-C67085, and AR-C66096. The aim of the present study was to determine whether externally added ADP could still act through the Gi pathway in Gαq-deficient mouse platelets and thereby amplify the residual responses to agonists such as thrombin or collagen. It was found that (1) ADP and adrenaline still inhibited cyclic AMP accumulation in Gαq-deficient platelets; (2) both agonists restored collagen- but not thrombin-induced aggregation in these platelets; (3) the effects of ADP were selectively inhibited in vitro by the ATP analog AR-C69931MX and ex vivo by clopidogrel and hence were apparently mediated by the P2cyc receptor; and (4) high concentrations of ADP (100 μmol/L) induced aggregation without shape change in Gαq-deficient platelets through activation of P2cyc. Since adrenaline was not able to induce platelet aggregation even at high concentrations, we conclude that the effects of ADP mediated by P2cyc are not restricted to the inhibition of adenylyl cyclase through Gi₂.