The increase in activities of hepatic thymidylate synthetase (EC 2.1.1.45) and thymidine kinase (EC 2.7.1.21), which catalyse the formation of thymidylate through the de novo and salvage pathways, respectively, were significantly suppressed during liver regeneration in rats which were given glucocorticoids (hydrocortisone and dexamethasone) or indomethacin. These drugs also prevented the augment of hepatic DNA content in 24 h regenerating liver.