The effects of variable degrees of liver regeneration induced by two-thirds partial hepatectomy or intoxication with different doses of carbon tetrachloride (CCl4) on increased production of alpha foetoprotein (AFP) have been studied in very young (5-6 weeks) and older (15 weeks and more) rats by counter-immunoelectrophoresis (sensitivity down to 250 ng/ml). In the young animal adequate regeneration following two-thirds hepatectomy as well as 100 myl CCl4 successfully induced large increases in serum AFP levels. Smaller doses as well as a large fatal dose of the toxin, all of which resulted in inadequate regenerative activity, failed to excite AFP synthesis and secretion up to detectable levels. The adult animal did not show detectable AFP with any of these procedures. However, necrosis induced in the regenerating adult liver 24 h following partial hepatectomy did result in the detection of small amounts of protein in the serum. It is concluded that in hepatic regeneration of this species, the age of the animal as well as the magnitude of regenerating activity in the liver are critical in the occurrence of high levels of serum AFP.