Primary hyperparathyroidism (HPTH) is frequently associated with hypertension. To date, the relationship between these two conditions is still not clear. We have studied 34 consecutive patients with primary HPTH due to a parathyroid adenoma. The diagnosis was later surgically confirmed in 32 cases. Ten of thirty-four HPTH patients were hypertensive. Before adenomectomy (PTHX) and 1-2 months after PTHX, we measured the following parameters in all patients: circulating levels of total and ionized Ca, intact immunoreactive parathormone (iPTH) (1-84), plasma renin activity (PRA), aldosterone, and daily total urinary catecholamine excretion. Moreover, 10 hypertensive HPTH patients, 10 normotensive HPTH patients, compared to 10 to 10 sex- and age-matched healthy normotensive subjects, underwent an acute norepinephrine test to assess vascular reactivity to a pressor agent. Before PTHX, no significant difference was observed between normotensive and hypertensive patients in all the above-mentioned variables, except for PRA and plasma aldosterone levels which were higher in hypertensive patients. Furthermore, the pressor response to the norepinephrine test was significantly greater in hypertensive HPTH patients than in the other 2 groups. After PTXH, serum Ca and intact iPHT (1-84) levels were reduced to normal values in all patients, while blood pressure, PRA and plasma aldosterone levels became normal in 8 of 10 hypertensive patients. The pressor response to the norepinephrine test was similar in the 2 groups. These results are consistent with the hypothesis of a direct effect of PTH on renin secretion which could contribute to the pathogenesis of hypertension and to the vessels sensitization to pressor agents.