We studied a 25-year-old man suffering from primary orthostatic hypotension whose blood pressure decreased to 65/45 mm Hg during orthostasis and to 95/70 mm Hg during ergometric exercise (50 and 100 watt), and whose heart rate responses were inadequate. Resting catecholamine levels were within the normal range and did not show any significant increase related to orthostasis or to ergometric exercise. Hypersensitivity was observed to low doses of intravenous noradrenaline and isoproterenol. Specific binding of 3H-Yohimbine to intact platelets revealed a normal number of alpha-2-adrenoreceptors in agreement with the adrenaline-induced platelet aggregation in vitro, which was, however, in contrast to hypersensitivity to noradrenaline. Specific 3H-Dihydroalprenolol binding to intact polymorphonuclear leucocytes revealed an increased beta-2-adrenoreceptor density in agreement with hypersensitivity to Isoproterenol. Prescription of Fludrocortison improved orthostatic hypotension.