The high circulating concentrations of immunoassayable parathyroid hormone observed in chronic renal failure are due to a number of factors. These include altered metabolism of the hormone and also end-organ unresponsiveness which may, indirectly, cause increased secretion of parathyroid hormone. The response of the overactive parathyroid glands to changes in plasma calcium and magnesium is variable and caution is needed in evaluating the suppressibility of parathyroid hormone secretion in acute studies. 1alpha-Hydroxylated derivatives of vitamin D can effectively suppress parathyroid gland overactivity. This effect may not necessarily be medicated through hypercalcaemia and vitamin D metabolites may act directly on the parathyroid glands.