The effect of intravenous administration of homologous fibrin degradation products and thrombin on fibrinogen synthesis was assessed in rabbits. The relative fibrinogen synthesis rate was calculated as a ratio of the amount of radiolabelled lysine incorporated into fibrinogen to the amount incorporated into albumin during the same measurement period. An increase in this ratio above control would indicate a relatively specific stimulation of fibrinogen synthesis as compared with albumin, which is not an acute-phase reactant. Injection of 45 mg of 'early' or 'late' fibrin degradation products failed to produce a significant increase in the relative fibrinogen synthesis rate, suggesting that fibrin degradation products play no feedback role in controlling fibrinogen synthesis. Infusion of small amounts of homologous thrombin (15--25 NIH u) was followed by a small but statistically significant elevation of the relative fibrinogen synthesis rate. This was not accompanied by any increase in the levels of fibrinogen degradation products in plasma, or by any decrease in plasma fibrinogen concentration, possibly suggesting that thrombin can stimulate fibrinogen synthesis by a mechanism independent of significant fibrinogenolysis or intravascular coagulation.