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Regenerating liver accumulates lipid for about 20 hr following partial hepatectomy. During this time incorporation of intravenously administered palmitate-9,10-3H into βlipoprotein increased. 13 hr after partial hepatectomy, there was no change in the level of serum β-lipoproteins, but the specific activities of the triglycerides in the liver and β-lipoproteins were significantly diminished. Extension of these studies to the isolated perfused liver system demonstrated that 13 hr after partial hepatectomy the regenerating liver is capable of secreting greater quantities of the lipid, but not the protein, moiety of the β-lipoproteins in comparison with liver taken immediately from a partially hepatectomized animal, although there was no difference between the weights of the livers. However following addition of palmitate-3H and 14C-labeled amino acids to the perfusate, the specific activity of the hepatic and β-lipoprotein triglycerides of the liver excised 13 hr after partial hepatectomy was diminished, but that of the protein was not affected. Prelabeling of the accumulated triglyceride with palmitate-1-14C in vivo revealed that the proportions of the accumulated triglyceride secreted as β-lipoproteins by perfused livers excised immediately and 13 hr after partial hepatectomy were identical. It is concluded that regenerating liver rapidly acquires the ability to mobilize triglycerides at a rate equal to that of the much larger normal liver, so that it can handle all free fatty acids presented to it.