We have investigated the mechanisms leading to platelet aggregation following thrombin interaction with the glycoprotein (GP) Ib-IX-V complex. We show that platelets desensitized for the two thrombin receptors, PAR-1 and PAR-4, are still able to aggregate in response to thrombin and that this aggregation can be inhibited by a monoclonal antibody (VM16d) that blocks thrombin binding to GPIbalpha, or by pretreatment of platelets with Mocarhagin, a protease that specifically cleaves GPIbalpha. The thrombin/GPIbalpha-initiated signaling cascade induces platelet shape change through activation of the Rho kinase p160ROCK, independent of calcium mobilization, transient MEK-1 phosphorylation as well as the cleavage of talin through a calcium-independent mechanism. This signaling cascade does not induce the exposure of high affinity alphaIIbbeta3 integrin receptors, nor does it lead to micro -calpain cleavage of filamin or the integrin cytoplasmic tail. In contrast, we provide evidence that binding of thrombin to GPIbalpha induces fibrin binding to resting alphaIIbbeta3 leading to fibrin-dependent platelet aggregation and clot retraction, that can be selectively inhibited by alphaIIbbeta3 antagonists such as RGDS, the dodecapeptide or lamifiban, as well as by the fibrin polymerization inhibitor GPRP-amide.