1. Thyroparathyroidectomy (TPTX) carried out at 72 h before partial hepatectomy (PH) reduced the induction of hepatic thymidylate synthetase (TS) and thymidine kinase (TK), which are rate-determining enzymes in DNA synthesis, at 24 h after PH.
2. When TPTX was carried out at 24 h before PH, TK activity at 24 h after PH was not reduced at all, yet TS activity was reduced significantly. Thus the effect of TPTX differed in time dependence between TS and TK.
3. The depression of TK activity in rats which were subjected to TPTX at 72 h before PH, was recovered by Ca2+ supplementation. This result demonstrated that the rise of TK activity in regenerating liver is regulated by plasma Ca2+.
4. Since a high dose of tri-iodothyronine (T3) was required to cause elevation of the activities of these enzymes and DNA content in 24 h-regenerating liver of TPTX rats, the relative contribution of T3 to liver regeneration may be small.