The different components of the baroreceptor reflex were studied in 8 patients with idiopathic orthostatic hypotension. The reflex arc was interrupted in all patients and the lesion was most probably localized to the efferent sympathetic fibres. This was suggested by the negative cold pressor test, the absence of pressor response to mental arithmetic, and loss or reflex sweating. Selective sparing of sympathetic vasodilator fibres was probable in 2 patients. Cardiac acceleration after atropine was less than in normal subjects suggesting some degree of cardiac denervation. The present study indicates that lesions in efferent sympathetic fibres are a common cause of idiopathic orthostatic hypotension. Other components of the reflex arc were intact in this series of patients.