1. Fibrinogen catabolism was studied in ten rabbits before and after suddenly elevating the intravascular pool by injecting a single dose of homologous fibrinogen equivalent to 18–83% of the corresponding protein already present in this pool. Of the two metabolic tracers used, [131I]fibrinogen was injected 3–4 days before and [125I]fibrinogen at the time of introducing the change.
2. Analysis of total body as well as of the circulating radioactivities indicated that the excess protein did not affect the fractional catabolic rate constant, thus permitting the conclusion that fibrinogen catabolism closely follows a first order kinetic reaction. In some animals a fraction of the injected fibrinogen shifted to extravascular sites and altered the pool ratio. Elimination of the excess protein took about 5 days and was accompanied by changes in the synthesis rate of fibrinogen.