The effect of plasma parathyroid hypertensive factor (PHF)from 153 essential hypertensive subjects on pressor response in normotensive rats was evaluated and plasma levels of parathyroid hormone (PTH) in these patients were measured. The results showed that PHF from 61 patients could induce a delayed increase in blood pressure in the normotensive rats. Mean arterial pressure (MAP) began to increase at 10 minutes after injection and a peak response was reached at 50 minutes while pre-injection MAP level was recovered at 70 minutes. On the other hand, plasma PHF in remaining 92 patients failed to induce a pressor effect. Plasma PTH in the 153 patients were considerably higher than those of the control, and the plasma PTH from the two groups, either with or without PHF, showed significant difference in comparison with the control, but plasma PTH between these two groups did not show significant difference. The results have suggested that pressor effect was independent of PTH but depends on PHF.