Abstract. Since calcitonin stimulates calcitriol production we examined whether a feedback regulatory mechanism exists between these hormones. First, the calcitriol receptor was localized immunocytochemically in cultured TT cells originating from a human thyroid medullary (C-cell) carcinoma. The receptor was demonstrated by light and electron microscopy in cell nuclei and in small amounts in the cytosol. Receptor levels increased when the cells were cultured at physiological (120-240 pmol/l) or somewhat higher concentrations (1200-2400 pmol/l) of calcitriol. In parallel, the same doses of calcitriol markedly inhibited secretion of calcitonin into the medium. These results indicate a feedback loop between calcitriol and calcitonin.