The present studies investigate the effects of glucocorticoids on the function of the parathyroid glands using primary cultures of bovine parathyroid cells. Treatment of parathyroid cell cultures with dexamethasone for 48 h caused a dosedependent stimulation of PTH secretion. The minimal concentration of dexamethasone required for a significant stimulation of PTH secretion was 0.1 nM. The stimulatory effect of dexamethasone on the secretion of PTH was found within 12 h of treatment with 100 nM dexamethasone. The steroids deoxycorticosterone and cortexolone, which do not have glucocorticoid activity were without effect of PTH secretion. Since glucocorticoids may modulate the effects of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] in other tissues, additional studies were performed to evaluate the interactions of glucocorticoids and 1,25-(OH)2D3. Addition of 1,25-(OH)2D3 to parathyroid cell cultures for 48 h significantly suppressed PTH secretion. In the presence of dexamethasone, however, 1,25-(OH)2D3 also significantly decreased PTH secretion, although it did not reduce PTH secretion to control levels. The treatment of parathyroid cell cultures with 100 nM dexamethasone did not affect the parathyroid cell content of 1,25-(OH)2D3 receptors. In summary, these studies indicate that glucocorticoids significantly increase the secretion of PTH in vitro. This stimulatory effect can be inhibited by 1,25-(OH)2D3. The parathyroid gland is an additional site of physiological antagonism of glucocorticoids and 1,25-(OH)2D3.