Hydrocortisone sodium succinate (65 mg/kg body weight), Zn-ACTH (40 U/kg) and the intraperitoneal injection of Celite (200 mg/kg) decrease the incorporation of 14C-thymidine into the DNA of regenerating rat liver by about 60%. This decrease is not followed by a corresponding inhibition of cell division. The agents applied at the end of G1 phase or in the S phase of the cell cycle probably change thymidine metabolism and the observed decrease of thymidine incorporation does not represent true inhibition of DNA synthesis. The experiment with regenerating liver slices has shown that this disproportion is partly caused by decreased 14C-thymidine transport into the cells.