Since release of PTH from parathyroid adenomas is stimulated by hypocalcemic perfusions, we propose to perform serial venous sampling from a catheter positioned in the SVC to detect abrupt elevations of PTH following the injection of contrast agent into each of the vessels selectively catheterized at the time of parathyroid arteriography (superior thyroid arteries, thyrocervical trunks, internal mammary arteries). Samples from the SVC catheter will be obtained at 20 second intervals up to one minute and in some patients simultaneous peripheral samples. An elevation of parathyroid hormone 1.5 times above the baseline will be considered diagnostic. This technique will be applied routinely at each injection of standard parathyroid arteriography even in the absence of an angiographically-visualized adenoma: an elevation of PTH will provide localizing information comparable to venous sampling. In addition, when any of the imaging studies suggest that a particular anatomic site is the likely location of the patient's adenoma, sodium citrate, a calcium chelating agent, will be injected into the artery supplying this region. Blood samples will be obtained in similar fashion to those samples taken with contrast injections to measure PTH release following sodium citrate injection and determine whether this is a more effective hypocalcemic stimulant.