The purpose of this study was to clarify the pathophysiology of primary hyperparathyroidism by looking for differences between parathyroids from eucalcemic patients and patients with primary hyperparathyroidism (HPT) with respect to the following parameters: intracellular parathyroid content of parathyroid hormone (PTH) and parathyroid hormone messenger RNA (PTH mRNA); and serum PTH and calcium levels of patients and patient age. Coded samples of human parathyroid biopsies were assayed for PTH content with a C-terminal-specific radioimmunoassay. Total cellular RNA was extracted, and PTH mRNA was quantified by dot-blot analysis. These results were tabulated along with associated data on patient age, preoperative serum PTH, and preoperative calcium levels. The content of PTH was significantly higher in true normal (TN) parathyroids than in parathyroids from patients with hyperparathyroidism. PTH content of adenomas and hyperplasias were similar. PTH content of normal parathyroids biopsied from patients with parathyroid adenomas (NA) was statistically higher than that of adenomas but statistically lower than that of TN parathyroids. PTH mRNA and PTH content were correlated (p < 0.001) for TN parathyroid glands; however, it was not true for glands (grossly normal or otherwise) in patients with HPT. Patient groups were similar with regard to mean patient age and intracellular PTH mRNA levels. Hypercalcemic patients were similar with regard to preoperative serum calcium and PTH levels. NA parathyroids, adenomas, and hyperplasias are different from TN parathyroids with regard to their PTH content. PTH mRNA was similar across all groups. The relation between intracellular PTH mRNA and PTH was significantly absent in patients with HPT compared with TN glands. Furthermore, we have found that PTH content of normal parathyroid in patients with adenoma is similar to that of hyperplastic and adenoma tissues. These data suggest that the PTH content of parathyroid tissues may be of use in differentiating normal from abnormal parathyroids.