1. At 0.5–1.0h after partial hepatectomy the intracellular acid-soluble fraction of rat liver took up twice as much radioactivity from [3H]orotic acid, [3H]uridine and [3H]thymidine as did similar fractions from sham-operated animals. This increase in penetration was not prevented by adrenalectomy or actinomycin, both of which decreased precursor uptake into nuclear RNA at this time. The increase in entry was still shown by thymidine at 22h after operation, at the height of the S period. Adenine penetration was not increased 1h after partial hepatectomy. 2. Plasma concentrations of ornithine and possibly methionine, tyrosine and lysine were raised 1.5h after partial hepatectomy. [3H]Lysine entry into regenerating liver at this time was increased by 60% [3H]valine uptake was unaffected. Intracellular amounts of tyrosine, phenylalanine and ornithine in the liver were also increased. 3. The relation of these events to the start of liver regeneration is discussed.