Many physiological abnormalities have been described in essential hypertension, yet the cause of this condition remains unknown. Included among the reported abnormalities are alterations in serum and tissue calcium levels, abnormalities in calcium regulating hormones, and the involvement of the parathyroid gland in some forms of hypertension. In the current study, the authors review evidence suggesting that a newly described hypertensive factor may explain a number of these abnormalities. This factor was first described in spontaneously hypertensive rat (SHR) plasma and is characterized by its ability to raise blood pressure in a delayed manner in normotensive rats, as well as by its ability to increase calcium uptake in vascular smooth muscle. The factor seems to be produced by the parathyroid gland, yet it is distinct from parathyroid hormone. Histological studies suggest that the factor may be produced by a specific cell type in the parathyroid glands. Given the parathyroid gland dependency of this factor, the authors have tentatively named it "parathyroid hypertensive factor," or "PHF."