Secretion of PTH is regulated by extracellular calcium via calcium receptors (CaR) on the parathyroid cell surface. Recent studies have shown a decreased expression of CaR messenger RNA (mRNA) and CaR protein in pathological parathyroids. We studied the expression of CaR mRNA in pairs of adenoma and adenoma-associated normal gland from the same patients (n = 17) and in biopsies of normal parathyroid glands of normocalcemic subjects (n = 4) using in situ hybridization with oligonucleotide probes on frozen sections. No down-regulation of CaR mRNA caused by hypercalcemia could be demonstrated in the normal adenoma-associated parathyroids when compared with the normal parathyroids of normocalcemic subjects. In contrast, CaR mRNA in the adenomas was significantly reduced to 64% (median; range 41-98) of the corresponding normal adenoma-associated glands. No correlation was seen between CaR mRNA in the adenoma and preoperative serum calcium, PTH, or weight of the adenoma. Loss of heterozygosity studies were performed on adenomas using markers for the locus of the CaR gene on chromosome 3q. No allelic loss was demonstrated, excluding allelic loss as the cause for decreased CaR mRNA expression in the adenomas. It is concluded that the lowered levels of CaR mRNA in parathyroid adenomas may contribute to the increased set point of PTH secretion. In large adenomas the increased cell mass seems to be more important for the increased secretion of PTH.