The parathyroid gland is unusual among endocrine and other secretory tissues in that increased concentrations of Ca in the extracellular fluid inhibit rather than facilitate or stimulate hormone secretion. To determine whether translocation of Ca2+ across the plasma membrane might be involved in the secretory processes of the parathyroid gland, the effects of the calcium ionophores A23187 [2(3.beta.,9.alpha.,11.beta.-trimethyl)-8-(2-pyrrole-carboxymethyl)-1,7-dioxaspiro [6.6] undecyl-2.beta.-methyl-5-methylaminobenzoxazole-4-carboxylic acid] and X537A [.beta.-diethylaminoethyldiphenylpropyl acetate hydrochloride] on the secretion and biosynthesis of parathyroid hormone (PTH) were studied in vitro. Slices of bovine parathyroid glands were incubated for 3-4 h with radioactive leucine. Secretion of radioactive and immunoreactive PTH by the gland slices was evaluated by measurements of acid-insoluble radioactivity and radiolabeled PTH by polyacrylamide-gel electrophoresis and by radioimmunoassay, respectively. Biosynthesis of PTH was evaluated by electrophoretic analysis of radiolabeled proparathyroid hormone (ProPTH) and PTH in evtracts of gland slices that had been pulse-labeled for 25-30 min with [3H]leucine and by specific radioimmunoassay of amounts of immunoreactive ProPTH in extracts of the gland slices. At concentrations of 1 .mu.M or less, the ionophores inhibited significantly the secretion of both radiolabeled and immunoreactive PTH independently of any demonstrable effects on hormone biosynthesis. Inhibition of PTH secretion by ionophore A23187 did not occur when Ca and Mg was omitted from the incubation media. Higher concentrations of ionophores (10 .mu.M), in addition to inhibiting hormone secretion, specifically blocked the cellular conversion of ProPTH to PTH without observable effects on the synthesis of total cellular protein. EM examination of the ionophore-treated (X537A) tissues revealed striking ballooning and disruptive changes in the Golgi complex and shrinkage of the mitochondria. Cellular influx of Ca leads to inhibition of PTH secretion. The Golgi complex appears to be involved in the intracellular conversion of ProPTH to PTH.