OBJECTIVES: Calcitonin (CT) has a regulative function in calcium (Ca) transfer from mother to the fetus. There are indications that cholecalciferol can play a role in this process. With this background, the effect of 24, 25-dihydroxycholecalciferol (24, 25-DHC) on immunoreactive CT by human placenta at term was studied in vitro. METHODS: Placental tissue samples were incubated in culture medium with normal total Ca (1.3 mM) concentrations during 6 hours at 37 degrees C. 24, 25-DHC was added to the samples. RESULTS: Addition of 24, 25-DHC in concentrations of 25, 75 and 150 pg/ml was followed by a significant decrease of CT levels in medium. The concentrations decreased from 100% (basis value after 2 hours) to 31%, 46% and 49% respectively. CONCLUSION: Probably 24, 25-DHC has a modulating effect on the regulation of placental Ca transfer by CT.