The purpose of this study was to investigate baroreflex activity in hypertensive patients with orthostatic hypotension (OH) due to sinoaortic baroreceptor denervation. The study concerned 3 patients (pts), 58-63 years, mean age 60.6 +/- 2 with both arterial hypertension (paroxysms recorded at 250/130 mmHg) and OH. They received radiation therapy to the entire cervical area for neoplasm, 9.6 +/- 2.8 years ago and had a carotid murmur without significant stenosis. Every pt had a severe and symptomatic OH: blood pressure (BP) and heart rate (HR) were respectively 163 +/- 17/105 +/- 7, 82 +/- 5 b/mn in lying position and 82 +/- 16/53 +/- 9 mmHg, 99 +/- 1 b/mm in standing position. The standing-induced increase in HR was lower (delta HR = + 17.3 b/mn) than expected; atropine (0.02 mg/kg) infusion and cold pressor test were ineffective; the massage of sinocarotid receptors induced a slight decrease in HR (delta HR = - 8 b/mn) and BP was not modified by Valsalva's maneuver. Infusion of norepinephrine (0.016 mg/mn) performed in one pt, increased BP without effect on HR. Platelet alpha 2-adrenoreceptors (alpha 2AR) evaluated by (3H) Yohimbine binding showed a significant increase in alpha 2AR number (Bmax), without any significant change in affinity (KD) when compared with normotensive and essential hypertensive pts: (table; see text) This study described an unusual etiology of a paroxysmal hypertension with orthostatic hypotension, demonstrated the impairment of baroreflex activity and suggested the potential interest of platelet alpha 2 adrenoceptors measurement to evaluate sympathetic tone in these patients.