: OBJECTIVES: Thymidine kinase (TK, EC 184.108.40.206) is a part of the pyrimidine salvage pathway, involved in DNA synthesis. In turn, adenosine kinase (AK, EC 220.127.116.11) functions as a part of the purine metabolic pathway, involved in DNA synthesis. Melatonin (Mel) is an indoleamine which is known to inhibit growth processes in the thyroid gland and also in other endocrine and non-endocrine tissues. The aim of our study was to examine TK and AK activities in homogenates of the rat thyroid lobes remaining after contralateral hemithyroidectomy (hemiTx); additionally, incubations with Mel (10(-6), 10(-9), and 10(-12) M) were performed. METHODS: The experiment was performed on young male Wistar rats (6-week old). The enzyme activities were measured by ascending chromatography and expressed as the amounts of radioactive reaction products of the phosphorylation of dThd (for TK) and of dAdo (for AK). RESULTS: 1. HemiTx increased TK activity in homogenates of the remaining thyroid lobe; 2. Mel increased TK activity in all the groups (intact, sham-operated- and hemiTx-rats), except for the concentrations of 10(-9) and 10(-12) M in the hemiTx-rats, in which the increasing effects of Mel on TK activity reached the borderline statistical significance only; 3. Mel increased the AK activity in intact and in shamTx animals; 4. No statistically significant changes were found in AK activity following Mel in vitro in the incubated remaining thyroid lobes, collected from hemiTx-rats.