Although noninvasive diagnostic studies [ultrasound (US), computed tomography (CT), sestamibi scanning, magnetic resonance imaging (MRI)] detect parathyroid pathology in one-half the patients referred to NIH with recurrent or persistent hyperparathyroidism (1), the invasive studies (direct aspiration for PTH, arteriography, venous sampling) must be performed in the remaining patients to provide definitive preoperative localization (2). Arteriography is positive in less than half the patients who must then proceed to parathyroid venous sampling, a technique requiring broad experience and yielding positive, but imprecise results as regards localization. The abrupt lowering of calcium in blood perfusing a parathyroid adenoma should stimulate the release of PTH which could be detected by simultaneous venous sampling. This principle, the intraarterial injection of a secretagogue with subsequent sampling for released hormone, has been applied successfully in the localization of insulinomas (3) and gastrinomas (4). In fact, portal venous sampling is no longer performed in this group of patients, having been completely replaced by the intraarterial injection of secretin or calcium as appropriate secretagogues for gastrin and insulin.