Cimetidine has been shown to lower circulating PTH levels effectively by causing an intraglandular rearrangement of PTH peptides in human parathyroid tissue in instances of primary hyperparathyroidism. A state of "euparathyroidism" is established. It has been statistically found at a 98.7 per cent confidence level that preoperative treatment with cimetidine effectively prevents symptomatic hypocalcemia postoperatively after parathyroid operations. It is anticipated that cimetidine has provided a tool for the in depth exploration of parathyroid disease.