The rate of .alpha.-fetoprotein (AFP) catabolism in CCl4-treated rats was estimated to be essentially the same as that found in control adult and young rats. The blockade of the RES by carbon particle resulted in slightly shorter t1/2 [half life] values of AFP. The amount of AFP synthesized per day in CCl4-treated rats was theoretically calculated to be nearly 10-fold increase over that in control rats. The elevation of serum AFP concentrations in CCl4-treated rats is derived from the increased synthesis of AFP by injured liver.