In this study we report the presence of peripheral benzodiazepine receptors (PBRs) in human parathyroid glands and describe the effect of their benzodiazepine type ligands on parathyroid cell function. PBR binding features in normal parathyroid tissue were characterized and compared to parathyroid adenoma, using a specific and selective ligand for PBR, [3H] 1-(2-chlorophenyl)-N-methyl-N-(1-methyl-propyl)-3-isoquinoline-carboxamide ([3H]PK11195). Affinity and density of [3H]PK11195 binding sites in homogenate membrane preparations from adenomatous and normal tissues were determined. Parathyroid adenoma showed a statistically significant 2.2 fold increase of [3H]PK11195 binding sites, while the affinity remained unchanged. Our results represent the first evidence of PBRs in parathyroid glands and suggest for them a role in influencing PTH release. A clear trend of PBR up-regulation in parathyroid adenoma was also found.