1. At 1h after operation livers from partially hepatectomized rats showed a 60–100% increase in the capacity to concentrate 3H radioactivity from orotate, thymidine or uridine with respect to the radioactivity in plasma. Uptake of [3H]cytidine into liver was unaffected, as was entry of any precursor studied into any tissue other than liver. 2. This increase in intracellular radioactivity was detectable 10min after operation with both orotate and thymidine. With orotate the augmentation had disappeared by 3 days, but with thymidine it was still evident 8 days after partial hepatectomy, when [3H]thymidine incorporation into DNA was no longer increased. Competition studies established that orotate was not entering the liver by the same mechanism as thymidine. 3. In the soluble fraction of the liver all the 3H radioactivity from orotate was present as uridine nucleotides. Thymidine was not phosphorylated, and was believed to be catabolized.