Depressed expression of calcium receptor in parathyroid gland tissue of patients with hyperparathyroidism. The factors involved in abnormal parathyroid cell secretory function and growth in patients with primary (I°) and secondary (II°) hyperparathyroidism are still incompletely understood. We compared the expression of the calcium-sensing receptor (CaR) at the gene message and the protein level in parathyroid tissue obtained from patients with I° non-uremic or II° uremic hyperparathyroidism with that in normal parathyroid tissue, using in situ hybridization and immu-nohistochemistry techniques. The expression of the CaR mRNA and protein was reduced in most cases of I° adenoma and II° hyperplasia, compared with strong expression in normal parathyroid tissue. In II° hyperparathyroidism, expression of both receptor mRNA message and protein was often particularly depressed in nodular areas, compared with adjacent non-nodular hyperplasia. Decreased Ca-R expression in adenomatous and hyperplastic parathyroid glands would be compatible with a less efficient control of PTH synthesis and secretion by plasma calcium than in normal parathyroid tissue.