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1. At 1h after operation livers from partially hepatectomized rats showed a 60-100% increase in the capacity to concentrate (3)H radioactivity from orotate, thymidine or uridine with respect to the radioactivity in plasma. Uptake of [(3)H]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 [(3)H]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 (3)H radioactivity from orotate was present as uridine nucleotides. Thymidine was not phosphorylated, and was believed to be catabolized.

1. In regenerating rat liver the phosphate content of the lysine-rich histone F1, but not that of the more arginine-rich histone F3-1, increases during the period of DNA synthesis. 2. The phosphorylation of histone F1 in this ;S period' is decreased by gamma-irradiation, but, like phosphate uptake into DNA, is affected to an even greater extent if the irradiation is given in the presynthetic period. 3. Histones from three species of sea-urchin eggs show similarities to the F2 and F3 groups of histones from mammalian thymus gland. 4. The proportion of thiol to total thiol plus disulphide in acid extracts from sea-urchin eggs varies from less than 20% in mature unfertilized eggs to 59% just before cleavage. 5. The phosphorylated forms of histones F1 and F3 are less effective in decreasing DNA synthesis by DNA polymerase than the non-phosphorylated forms. 6. Oxidation of thiol groups on histone F3-1 does not affect its capacity to decrease DNA synthesis in vitro.