Dogs injected with 125I-fibrinogen were given: 1. a thrombin infusion during 1 hr, and 2. simultaneous infusions of epsilon aminocaproic acid (EACA) and thrombin, respectively. Platelet number, fibrinogen concentration and radioactivity in plasma, fibrinogen and serum were determined. Special comparative studies were made between one dog given thrombin and one given EACA together with thrombin. Samples obtained before and at 2 hrs after the start of the thrombin infusions were analyzed with respect to the radioactivity distribution in the blood and in plasma after Sepharose and Sephadex flitrations. Trials were made to identify the fibrinopeptides. The immunological behaviour of serum and of eluate fractions was studied. An occurrence of fibrinolysis in the dogs already before the thrombin infusions was indicated. After the thrombin infusions there was in plasma a wide variety of fibrinogen degradation products with the antigenic determinants of fibrinogen. No products with higher molecular weight than that of fibrinogen were demonstrable. When EACA was given together with thrombin there was an inhibition of degradation products. The results give further support for a secondary fibrinolysis in intravascular coagulation. No evidence was obtained for the occurrence of fibrinopeptides.