Thrombin induces a redistribution of glycoprotein (GP) Ib/GP IX complex from the platelet surface into the surface connected canalicular system (SCCS). This redistribution results in a reduced interaction of platelet GP Ib with von Willebrand factor (vWF) bound to subendothelium leading to impaired platelet adhesion. In this study we show that the platelet aggregation and degranulation require concentrations of thrombin above 0.05 U/ml, while the decrease in GP Ib function (about 50% of control value), as determined by ristocetin induced platelet agglutination, can be induced by lower concentrations (0.01-0.04 U/ml). Moreover, we show that when adding thrombin inhibitors to the platelets preincubated with < 0.04 U/ml thrombin for 5 min, their agglutinability by ristocetin was gradually recovered within 30 min, indicating that in these conditions the decrease in platelet adhesiveness is reversible. Immuno-electromicroscopic study showed that this restoration of platelet GP Ib function was associated with a reversed translocation of GP Ib from the SCCS to the plasma membrane. The data obtained from counting gold particles showed that the ratio of GP Ib immunolabelling on the external membrane versus that on the SCCS was 3.31 +/- 0.90 for resting platelets, down-regulated to 0.84 +/- 0.13 (P < 0.05 versus resting platelets) for the platelets treated with 0.04 U/ml thrombin and returned to 2.63 +/- 2.21 (P > 0.05 versus resting platelets) after incubation for 30 min with hirudin. However, the translocation of GP Ib was poorly reversed by thrombin inhibitors when higher concentrations of thrombin were used which induced platelet aggregation and large extent of degranulation. We conclude that thrombin affects platelets in a dose dependent manner, and that at low concentrations the decrease in platelet GP Ib related function is a reversible phenomenon.