A primary culture of mammalian parafollicular cells was established from rat thyroid glands in order to investigate the effects of serotonin and somatostatin on calcitonin secretion. Minced rat thyroid glands were dissociated with collagenase and cultured in a Ham's F-12K medium supplemented with calf serum (5%), insulin (1.3 X 10(-6) mol/l), hydrocortisone (10(-8) mol/l), transferrin (6.1 X 10(-9) mol/l), and glycyl-L-histidyl-L-lysin (2.5 X 10(-8) mol/l). Immunohistochemical peroxidase-antiperoxidase method revealed that the cultured parafollicular cells were immunopositive for human calcitonin, and electron microscopy demonstrated the existence of dense secretory granules in the cultured parafollicular cells. Addition of the Ca2+ to the culture medium stimulated calcitonin secretion from the cells dose-dependently as measured by radioimmunoassay. Pre-incubation of serotonin with the cells produced higher calcitonin levels in a dose-dependent manner. On the other hand, pre-incubation of somatostatin with the cells significantly inhibited calcitonin secretion.