The maintenance of a constant calcium ion concentration in the extra cellular fluid is essential for the functional welfare of many tissues of the body. Of the three principal hormones that affect calcium metabolism, the earliest to evolve were calcitonin and vitamin D. Their initial function may have been to conserve phosphate. With the development of terrestrial forms of life, they were probably adapted to regulate calcium homeostasis. To facilitate more efficient remodelling of the skeleton, parathyroid hormone (PTH) evolved later as an adjunctive control. In man, PTH appears to be the most important hormonal regulator of calcium ion homeostasis. Although the importance of the parathyroids to the vital economy of mineral metabolism has long been appreciated, during the past decade increased medical awareness concerning the frequency and clinical importance of abnormalities in calcium and bone metabolism has heightened interest regarding the diagnosis and pathophysiology of parathyroid dysfunction. The assessment of the integrity of the parathyroids must necessarily be based upon an understanding of the regulation of PTH secretion. Through the recent work of several investigators it is now apparent that multiple factors, not calcium alone, are potentially involved in regulation of PTH secretion. Although these newly recognized factors, which include a spectrum of peptides, biologically important amines, prostaglandins, cyclic nucleotides and magnesium (Aurbach, 1982), are capable of influencing parathyroid gland secretion, the primary physiologic regulator of PTH secretion is ionized calcium (Fujita et al, 1974; Brown et aI, 1976; Dietel et al, 1977; Hanley et al, 1978). Considerable advances in the understanding of the control of PTH secretion have confirmed the original observations of Patt and Luckhardt that linked parathyroid secretion inversely to the plasma content of calcium. The basic feedback mechanism or 'loop' by which parathyroid gland secretion is controlled thus differs significantly from the complex multihormonal cybernetic control systems typical of the thyroid and the steroid hormone-producing glands. In these latter glands hormone secretion is regulated by pituitary trophic peptides, which in turn