The effects of varying extracellular concentrations of calcium and magnesium were compared on the release of ³H-labeled and immunoreactive parathyroid hormone (PTH) by bovine parathyroid slices incubated for 4 h in vitro in media containing [³H]leucine. The quantity of [³H]PTH or immunoreactive PTH released was determined by polyacrylamide gel electrophoresis and by radioimmunoassay, respectively. Biosynthesis of PTH in response to extracellular concentrations of magnesium and calcium was evaluated by radioimmunoassay of amounts of proparathyroid hormone (ProPTH) in extracts of the parathyroid tissues and by electrophoretic analyses of [³H] ProPTH synthesized during a 35 min pulse-labeling of parathyroid slices. Rates of hormone release were found to vary in response to changes in concentrations of either calcium or magnesium. The effects, however, of these two cations on hormone release, although additive, were not equipotent. On a molar basis, approximately 2.5 times as much magnesium as calcium was required to produce equivalent secretory responses. Effects of the cations on hormone biosynthesis were small; magnesium had no effect and calcium altered synthesis by approximately 15%. Neither cation affected conversion of ProPTH to PTH. The results agree with those reported previously from studies of the effects on hormone secretion of calcium and magnesium in vivo and serve to emphasize that, within normal physiological concentrations of these two cations, calcium is the predominant regulator of PTH secretion and synthesis. (Endocrinology98: 197, 1976)