We examined the distribution of platelet fibrinogen and the exchange between intra- and extra-platelet fibrinogen in unstimulated and thrombin-stimulated platelets. In unstimulated platelets 60% of platelet fibrinogen was found in the soluble platelet fraction and 40% in the insoluble one. In platelets activated with thrombin, changes took place in the distribution of intraplatelet fibrinogen but not in the total fibrinogen content. At 0.5 U/ml of thrombin the fibrin(ogen) content of the insoluble and soluble fractions was approximately 80% and 20%, respectively. When we evaluated how extraplatelet fibrinogen affects the content and distribution of intraplatelet fibrinogen, we found that when unlabelled fibrinogen was added to unstimulated and thrombin-stimulated platelets the content and distribution of intraplatelet fibrinogen remained unaltered. However, when 125I-fibrinogen was added, it was incorporated into unstimulated and thrombin-stimulated platelets. In unstimulated platelets, 70% of the incorporated 125I-fibrinogen was in the soluble fraction and 30% in the insoluble. In thrombin-stimulated platelets the distribution of the incorporated 125I-fibrinogen was 62% and 38% in soluble and insoluble fractions respectively. MoAb to GPIIb–IIIa produced 80% and 60% inhibition of 125I-fibrinogen incorporation by unstimulated and thrombin-stimulated platelets. Our data showed dynamic exchange between intraplatelet and extraplatelet fibrinogen both in unstimulated and thrombin-stimulated platelets mediated mainly by GPIIb–IIIa.