The effect of somatostatin on plasma parathyroid hormone (PTH) was studied in 6 subjects with normal parathyroid function and one patient with a parathyroid adenoma. In 5 subjects, including the one with the parathyroid adenoma, plasma PTH was measured by radioimmunoassay during a 4-hour infusion of somatostatin (500 μg/h). In 2 subjects, PTH responses to EDTA were compared with those observed during a simultaneous infusion of somatostatin and EDTA. In no instance was there a discernible effect of the somatostatin infusion on plasma PTH. These results demonstrate that somatostatin does not suppress plasma PTH.