Platelet adhesion on von Willebrand factor (vWf) requires the co-ordinated adhesive function of glycoprotein Ib/V/IX and integrin alphaIIbbeta3. Recent evidence [Nesbitt, Kulkarni, Giuliano, Gonclaves, Dopheide, Yap, Harper, Salem and Jackson (2002) J. Biol. Chem. 277, 2965-2972] suggests that outside-in signals from both receptors play important roles in regulating platelet-adhesion dynamics under flow. In the present study, we have examined the mechanisms utilized by protein kinase C (PKC) to promote irreversible platelet adhesion on vWf. We demonstrate that PKC is primarily activated downstream of integrin alphaIIbbeta3, not glycoprotein Ib, during platelet adhesion on vWf. This integrin alphaIIbbeta3-dependent PKC activation establishes a positive-feedback loop that promotes further integrin alphaIIbbeta3 activation, calcium mobilization and firm platelet adhesion. This feedback loop appears to be most relevant at relatively low cytosolic calcium concentrations (mean Delta[Ca(2+)](i);100 nM) as artificially elevating calcium (mean Delta[Ca(2+)](i) > 500 nM) induces integrin alphaIIbbeta3 activation and irreversible platelet adhesion independent of PKC. Our studies demonstrate the existence of a complex signalling relationship operating between PKC, cytosolic calcium and integrin alphaIIbbeta3 that serves to regulate platelet-adhesion dynamics under flow. Furthermore, we have established the existence of PKC-dependent and -independent pathways regulating integrin alphaIIbbeta3 activation and stable platelet adhesion on vWf.