A new circulating hypertensive factor (parathyroid hypertensive factor; PHF) was shown to exist in the plasma of spontaneously hypertensive rats (SHR) but not in that of normotensive rats. PHF produced a delayed increase in blood pressure with a peak response at 45 min (bolus injection) or 60 to 90 min (continuous infusion). This increase in blood pressure was coupled with an in vitro increase in calcium uptake in rat tail artery with a similar time course. The involvement of calcium in the mechanism of action was supported by the inhibitory effect of calcium antagonists on the vascular action of PHF. Furthermore, PHF increased the intracellular free calcium concentration in cultured smooth muscle cells from rat tail artery. Parathyroidectomy and parathyroid transplant experiments indicated the parathyroid origin of PHF. A culture of parathyroid glands from SHR, but not from normotensive rats, produced in the medium a factor which has the same biological property and HPLC retention time as plasma PHF. A novel cell type was described in the parathyroid gland of SHR, but not normotensive rats, and the percent of these cells correlated significantly with plasma PHF level and blood pressure. In some hyperparathyroid patients, plasma PHF and hypertension were found, both of which disappeared after surgical removal of the parathyroid gland. In both animal models and human studies, PHF seems to be associated with low or normal plasma renin and salt-sensitive type of hypertension.