The location of a hyperactive parathyroid gland has been demonstrated preoperatively by means of isotopic labeling and external photoscanning. This has been accomplished with the use of gamma-emitting Se-75 selenomethionine as an amino acid precursor of parathyroid protein. Preliminary suppression of thyroid protein anabolism, to diminish regional radioactivity interference, was achieved by triiodothyronine administration. The potential clinical implications of this initial and successful localization of the parathyroid gland by the use of a radioisotope are considered.