The hemodynamic effects of clonidine were studied in four patients with sever idiopathic orthostatic hypotension and one patient with baroreceptor dysfunction. No depressor response to clonidine was observed in any patient with idiopathic orthostatic hypotension at any dosage. Rather, two patients responded to 0.4 mg of oral clonidine with a 40 mm Hg increment in systolic blood pressure lasting several hours. Each has been receiving clonidine, 0.4 mg twice daily, for one year with greatly increased functional capacity. The other two patients with idiopathic orthostatic hypotension had an even greater pressor response to 0.8 mg of oral clonidine, but adverse effects prevented continued therapeutic use. In marked contrast, the patient with baroreceptor dysfunction had a profound depressor response to 0.2 mg of clonidine. In the treatment of idiopathic orthostatic hypotension, the major advantage of clonidine over other pressor agents is its longer duration of action. The major adverse effects of the drug in these patients are sedation, dry mouth, altered mentation, and excessive hypertension. The drug should not be given to patients with mild idiopathic orthostatic hypotension or selective baroreceptor dysfunction, since severe hypotension may result.