The effects of increased ambient calcium concentration on parathyroid function in vivo and in vitro were assessed in each of 12 patients with primary hyperparathyroidism. Calcium injection and/or infusion were carried out preoperatively and parathyroid suppressibility was tested by the effect of calcium on nephrogenous cAMP (NcAMP). Dispersed parathyroid cells were then prepared after surgical excision of abnormal parathyroid tissue. Immunoreactive parathyroid hormone (iPTH) release was determined by RIA after incubation for 2 h at calcium concentrations ranging from 0.3-3.0 mM. Of nine patients with adenoma, iPTH release from dispersed parathyroid cells was inhibited less than 50% in 3 (26-40%); in these patients, NcAMP was suppressed 20% or less. In the remaining six patients, iPTH release in vitro and NcAMP release in vivo were inhibited 62-90% and 34-75%, respectively, by elevated calcium concentrations. In three patients with primary parathyroid hyperplasia, iPTH release was inhibited 75-81% and NcAMP release was inhibited 33-50%. Suppressibility of hormone release as a function of ionized calcium concentration generally showed agreement between in vivo and in vitro results for each patient. These results support the validity of these parameters, iPTH release from dispersed parathyroid cells and NcAMP, to assess secretory control in vivo and in vitro from abnormal parathyroid tissue. (J Clin Endocrinol Metab48: 604, 1979)